The price difference between the 2 Athlons in question is tiny compared to the price difference in the video cards! Its a $40 difference in cpus, versus a $140 difference in the price of a 7900 GT versus the 7600GT.
For gaming at higher resolutions a better video card is always better to have than a faster processor. In any case an Athlon 3200+ will EASILY overclock to 2,2 GHz, the same speed as the 3500+. However it also depends on the resolution of your monitor. You may not need a 7900GT if you have a 19" LCD at 1280 x 1024.
BTW - My sempron performs better than the Athlon 3500+ when running at 2.4 GHz, which is does with no sweat.