The Radeon 9100 doesn't really merit a lengthy description. Why? Because it's the exact same card as the Radeon 8500. There's no quabbling over performance or little difference here: they're the EXACT SAME CARD.
However, it's probably worth noting how the Radeon 9100 came about. The Radeon 9000, while a decent bargain card, couldn't match up to the Radeon 8500 in most tests; 8500, in the eyes of most people, was a superior card, if only slightly. The trouble is, "8500" sort of implies "worse than 9000." (This didn't stop nVidia from releasing the GeForce4 MX, a card inferior to the GeForce3. Rrrgh. Attack. Crush.) So ATi's marketing department, being composed of the sort of geniuses one could expect to see in the marketing department of a large company ("HyperZ," anyone?), came up with the idea for the Radeon 9100. The R200 chipset clearly had a bit of life left in it, so why not milk it, right?
The Radeon 8500 had regular and "LE" versions; the LE cards were just lower-clocked versions of the standard card (Standard card was 275/550, LE was 250/500) In the same way, the Radeon 9100 has "Pro" and regular versions, the Pro representing the regular Radeon 8500 and the regular representing the Radeon 8500LE. Apparently ATi is convinced this "Pro" thing is going somewhere.