The Radeon 9500, and its brother, the Radeon 9500 Pro, are based on the same design (R300) as the Radeon 9700 and Radeon 9700 Pro. (I wrote a little more about the GPU for these cards in Radeon 9700, and the only changes to the cards are below.) The 9500 and 9500 Pro are crippled, however, in rather inventive ways.
The Radeon 9500 Pro is just like the Radeon 9700 REGULAR; it has the same 275/540 clock (that's 275MHz core, 540MHz memory), but it also has a 128-bit memory interface (as opposed to 256-bit). This is more than enough to cool the card's heels, performance-wise, and it brings it to somewhere in-between the GeForce4 Titanium 4400 and GeForce4 Titanium 4600 in most regular benchmarks. However, the card is excellent at anti-aliasing and anisotropic filtering, so it usually pulls ahead of the entire GeForce4 line in that sort of benchmark by a long way. That's right-- downgrading the memory interface from 256-bit to 128-bit is enough for this decrease in performance.
The Radeon 9500 is identical to the Pro version, but has four rendering pipelines instead of eight. This seriously injures performance and makes the 9500 a less-than-perfect choice for most people aiming for a good price-performance spot.
But wait, there's more!
You see, the 9500 is really just a crude hack of the 9700. The Radeon 9500 non-Pro 128Mb shares the 9700 and 9700 Pro's PCB, L-shaped RAM and all. (The black ones don't, but that's why people don't buy the black ones for modding.) It is in fact possible to make a 9500 non-Pro into a 9700 non-Pro with a little software modding (copy-and-paste-and-pray), 256-bit memory interface and all. The 8 rendering pipelines seem to be software-disabled, though of course a few 9500s out there are going to have defective rendering pipelines (it's not like they're checking to see if all eight work, and besides, some of 'em are binned 9700s). This means that a user with a little time and a little luck (this isn't 100%, remember) can make a shiny Radeon 9700 for considerably less.*
UPDATE: ATi's stopped production. Why, you ask? Let me put it this way. They were selling a very, very powerful card (owners of the 9500 Pro, you made a very good choice!) that they hadn't crippled quite enough at midrange prices. And in the value sector, their Radeon 9500 non-Pro (which WAS crippled enough) could be made into a Radeon 9700 much of the time with just a little software work. Now they're trying to pitch the Radeon 9600 Pro, which is in fact a good deal slower than the 9500 Pro. Oh well. And now the damn thing's too expensive.. yeesh, $175 for a card this fast right now would be a good deal, but paying $175 for a card this fast several months ago? Again, if you bought a Radeon 9500 Pro, you made the right choice.
* This is pretty much copy and pasted from a Dan's Data letter, but I wrote that letter (and don't ask about the name, that's my failure to set up the name properly on my old email account), so I don't really feel bad about it.