The original ATI Radeon 8500 was clocked at 275/275, but various 3rd party manufacturers liked using cheaper parts, running it at lower speeds. ATI eventually released its own slightly lower-clocked version, called the 8500LE, at 250/250. The only difference is clock speeds, as far as I know. Note that 3rd party manufacturers still like to push it lower still, and I've seen some rated as low at 200/183!
The 9100 is just re-branded 8500LE, typically still running at 250/250 (like the Powercolor mentioned above). However, that varies ... the sapphire versions do indeed only run at 250/230 for the 64MB version and 250/200 for the 128MB. It's common to run the higher memory versions slower, to offset the greater manufacturing cost (and negate most of the performance gain, alas!)
I've just ordered a Retail ATI 8500LE 64MB from FutureShop here in Canada for approx. $65US after rebate. We'll see if it is the expected 250/250 when it arrives.
I would have liked the 128MB, but the cheapest I can find around here is the oem sapphire 9100 version (250/200 at best) for about $85US ... and I don't think it's worth an extra $20US for an oem card with twice the memory running 20% slower.
EDIT: Just an update, but the FutureShop retail ATI 8500LE is apparently clocked at 230/230 according to reviews on their website. It just showed up in the mail today, and I see it uses 4ns RAM, unfortunately. Ah well, still not bad for the price (I won't have time to install it until next week, but we'll see how it does then).