first of all are you going agp or pci-e?
if you plan to go pci-e, X700 and 6600 are your only choices
if you plan to go AGP, 9800pro and 6600 are yuor only choices.
performance wise, stock, the 6600 will be slowest, followed by the 9800pro and x700 in a tie.
no if you plan to do soem work and mod or oc your cards, it becomes completely different.
The 9800pros on sale now are basically all 9800XT cores and with the right bios flash and the flashrom utility, you re able to get a 9800xt out of it. I used a 128mb 9800xt bios and my radeon 9800pro is reconized asa 9800xt at overclocked core and memory speeds.
I score ~3300 3dmark2005s. Without the bios mod, but just a decent oc(my memory is a bad ocer, but my core is decent), I got about 3000 3dmark05s. Stock, expect ~2500-2600 3dmark05s
The 6600 is not moddable, but highly overclockable, but the memory which is the main bottleneck does not ooverclock so well. You can eexpect ~550mhz core oc from 300mhz, but the memory for the most part will not oc beyond 550 average. More than 600mhz on the memory is rare.
This limits the performance even oced quite a bit. It shouldnt be as fast as a 9800pro unless you watercool or get insainely lucky high memory overclocks. Stock, expect, ~2000 3dmark05s and ~2800 3dmarks overclocked.(~3100 oced at 500/700 which is the absolute highest i've ever seen for a 6600[NOT 6610 as those are basically underclocked 6600gts with a 6600 label. it even contains the sli connector!].)
the X700 is as good as the 9800pro stock but does not mod as well either. It ocs decent but not spectacular. expect 2500 3dmarks05 stock, ~2900-3000 3dmarks 05 oced.
of course these are average numbers with high quality/no optomizations set in driver settings, default benchmarks.
If you build new, go pci-e route and buy an x700 and be happy with it.
If you are upgrading a used computer with and AGP slot(the brown ones that are different from every other slot) buy an 9800pro, bios mod it and overclcok it(of course first check if it's an R360 core!)
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