The Radeon 9800 is a series of video cards produced by ATI. They are currently towards the high end, but have recently been replaced by the ATI x800 XT and the NVIDIA Geforce 6800 Ultra, though at the time of this writeup both of those cards are very expensive; this summer, they were going from between $600 and $900 in the open market, and while prices have dropped a bit since then, they are still quite expensive. The 9800 outperforms older NVIDIA cards by a significant margin in most benchmarks, and most hardware review sites consider it a good buy from a price/performance perspective.
The Technical Stuff
The Radeon 9800 series was first introduced by ATI in Q2 of 2003. Early versions were based on the R350 core, though later models, like the 9800XT, used what ATI called the R360 core, though it turns out the only changes were clock and memory speed increases. The R350 and R360 cores, both with a transistor count of 110 million, are manufactured with a 0.15 micron process by TSMC; ATI's other source for chips, United Microelectronics Corporation, currently only produces the older R200 series.
At the time of release, the cards were significantly faster than NVIDIA's offerings, and while NVIDIA later caught up with the release of the NV35, delays in production gave ATI a huge advantage. One place where this is most noticeable is, just like how DirectX 8 was based on the NV20, DirectX 9 is based directly on the abilities of ATI's R3x0 cores. Since Microsoft has stated that it does not plan on producing a new version of DirectX for a couple of years, this is quite a coup for ATI, and forces their competitors into spending time catching up with whatever ATI does, much like how AMD spent years just trying to reimplement the latest thing from Intel in their desktop CPUs.
One of the big new features in DirectX 9 is the use of fully programmable shaders. Until quite recently, video cards only provided certain hard-coded operations, or, at best, very limited programmability. Generally these operations matched up with what was commonly used in games and video playback. This was easy to implement, since all the operations could be hardcoded into an ASIC, but if a developer wanted to try out something new and interesting, they would have to do it on the host CPU, which not only hurt performance, but makes the graphics card less useful. Earlier cards often had small buffers for these operations, forcing the programmer to break up complicated sequences into multiple passes, which increased the number of transfers over the bus between the card and main memory. Since even AGP 8x can only handle a (theoretical) maximum of 2.1 GB per second, it's important to minimize these transfers whenever possible.
The R350 solves this problem with the F-Buffer, a FIFO built into the card itself. This allows the programmer to store intermediate results (up to 1024 of them) into a buffer. This is a major improvement over the R300's hard limit of 64, and removes one of the few advantages NVIDIA had over ATI, as the GeForce FX also supported this technique. Thanks to the introduction of new high-level shader languages like Cg, the programmer doesn't even have to be aware of the details, as long as the compiler knows how to exploit the abilities of the card.
Historically, ATI has been notorious for bad drivers, but within the last couple of years this situation has improved quite a bit. However, at this time there are no free software drivers for any version of the 9800 for users of XFree86/X.Org. There are closed source drivers from ATI which, based on user reports, work but are lacking in features and performance compared to the Windows drivers.
Radeon 9800 Pro: This was the first R350-based card, available in March of 2003. It had 128 megabytes of DDR memory and initially retailed for about $400 (currently priced at around $150-$200). The R350 ran at a clock speed of 380 MHz, and the memory bus ran at 340 MHz (though due to the use of DDR, this was usually advertised as being 680 MHz). A later version provided 256 megabytes of DDR-II memory, which has higher latencies but provides noticeably higher bandwidth. The base version can transfer over 20 gigabytes per second between the core and the onboard memory.
Radeon 9800: This is the same as the Pro, except with lower clock and memory speeds (325 MHz/290 MHz). For some reason the Pro version came out significantly prior to the "plain" 9800.
Radeon 9800 XT: This card is based on the R360, which is essentially just an R350 tweaked to run at higher speeds, in this case 412 MHz, with a 365 MHz memory bus. The circuit board in the reference implementation of the card was visibly redesigned from the Pro, and has about half a dozen more capacitors added. It currently sells for around $350.
Radeon 9800 SE: A cut down budget version of the 9800. There are two variants, the 128-bit and 256-bit versions. These bit sizes refer to the size of the memory bus. The 128-bit version has a 325 MHz core clock and memory speeds that seem to range from 250 to 300 MHz, depending on the card. The 256-bit version has a 380 MHz core clock and a 300-340 MHz memory clock. Both versions have only 4 pixel pipelines enabled, as compared to the 8 in the Pro and XT versions. One interesting thing to note is that the 256-bit version of the SE has the same specifications as the Pro, except with 4 of the pipelines disabled. Some clever hackers have discovered that on some of the chips, all 8 pixel pipelines are enabled and work, and are simply disabled by the drivers. So a simple software patch can give you a card with the performance of a 9800 Pro at half the cost. This is pretty hit or miss, since others have pipelines disabled due to manufacturing flaws, so you will quickly notice severe visual artifacts if the 4 additional pipelines are enabled on some SE cards. And of course, it will void your warranty.
Radeon 9800 Platinum Edition: Identical to the Pro, but with the 256-bit memory bus replaced by a 128-bit memory bus, and the bus itself runs at only 220 MHz. The core continues to run at 380 MHz, and for the price, less than half of a Pro, it remains a pretty good performer. In particular, it gives ATI a relatively inexpensive card that still provides a DirectX 9 compatible interface. (This seems to be a Power Color specific model).
All versions provide an AGP interface, and require a 4-pin Molex connector to provide additional power, rather than using the AGP Pro interface to provide additional juice, since support on motherboards for AGP Pro is somewhat spotty. A later revision of the R350, the RV380, is used in ATI's x600 cards, which use a PCI-E interface. ATI makes some of the cards, but numerous third parties, including Abit, Power Color and Sapphire also manufacture R350-based cards.
Note to the reader: I am not an expert in graphics or graphics cards. While I've done a bunch of reading for this writeup, it is entirely possible there are errors or misconceptions. If you see something wrong, please /msg me to let me know, and I'll fix it.