Your need for 2048 x 1536 @ 85 Hz does indeed limit you. My Radeon 8500LE 64MB (built by ATI) supports that refresh rate at that resolution, but my Radeon 9500Pro (again, built by ATI) only supports 60Hz at that resolution. I don't know about the 9600, but I doubt it would be superior to the 9500Pro.
As for speed differences between the cards, it depends on how you would use it. In a game like UT2003, the difference between the cards without any "eye-candy" (AA and AF) would be noticeable but not that great on your system (I'd estimate you'd get about 45-50 FPS in botmatch benchmark for your system with the 8500LE, and about 55-60FPS with the 9500Pro, based on my experience). The reason for this is that you are somewhat processor-limited there .... in other games, you would see the difference far greater.
The main difference with the 9500/9600 family is that you can crank up the AA and AF without much of a performance hit, relatively speaking, and these make games look much better. On the 8500/9x00 series of cards, you could still use AF without too much of a hit, but forget about any kind of AA. The implementation of AA is way too slow on the R200 core. And the AF doesn't look as nice either as compared to the later R300 cores (9500 and up). Also, if you ever upgrade your processor you will be hamstringed somewhat by the slower R200 core ... the newer R300s will scale up quite nicely with any future upgrades. So in that sense, if you are any sort of gamer, the 9600 (R300 core) will far surpass the 9200 (R200 core).
As for clock speeds of the 8500/9x00 series, the original 8500 ran at 275/275 (core/mem). The 8500LE ran (originally) at 250/250. Current built-by ATI versions (at least the 64MB) run at 230/230. As for the more common 9100 label, I've seen clocks range from 250/250 to 200/167! So, you have to watch out. Also, the 128MB versions are generally clocked slower on the memory (to offset the higher costs). For example, the Sapphire 9100 runs at 250/230 for 64MB version and 250/200 for the 128MB version.
The 9000/9200 series also runs at variable clock speeds (actually a bit higher on the Pros, but that's to compensate for the reduced pipelines, I believe). They come in Pro and non-Pro versions (don't even consider the non-Pro - it's just too slow). The net effect is that the 8500/9100 is still faster than the 9000/9200 Pros (although not by that much).
Your call ... it depends what matters to you the most, I guess. I find the 8500 runs well enough for current games on my son's system, but I wouldn't trade my 9500Pro on my system for anything

. Of course, I don't run at the resolution you want.