Depends mostly on your resolution. If its say 12x10 then the 512 doesnt offer much. Also, the 1950 pro would struggle at higher rez anyways with lots of eyecandy. The GTS 320 is a killer card, can handle anything with eyecandy up to 16x12 rez. You get what you pay for here. Theyre both the best bang for the buck cards out, just depends how much you wanna spend.
"How much am i willing to spend on a graphics card now?"
"How long do i want my card to last me? aka, 1 year..2 years..etc"
"what resolution do i want to play at?"
"Do i want to play my games with full AA and AF?"
answer those...then we will give u some extra help
since you've already considered the 8800gts 320mb, in this scenario the 8800gts would be a good buy in terms of insane gaming performance (at 1600x1200 reso or leass). Though in terms of sheer money, if you simply can't afford the 8800gts and you can't wait or can't get enough money in the extremely near future, the x1950pro would be a good enough buy. But i'd recommend a 256mb version rather, because simply most games today wouldn't utilize all 512mb, if it did the x1950 is already a tad bit slow to consider 512mb.