Hiya,
Yep on that p3 700 you won't really see a massive gain with a new GF4 etc
Still if you can get a gf#200/or larger series (none MX)

approx £110 here and upgrade cpu if poss you would see a benefit
I guess if i were in same shoes as you having a p3 mobo and wanting a later vid i would forego the vid and upgrade mobo/cpu/ram then at a later date (few mths sees gf4's prices dropping nice

)when possible (thinking is you maybe limited to upgradable cpu on present mobo)
with a faster mobo/cpu (ram if required) you will have a darn nice pc setup for a more modern vid
Hope this makes sense lol
Basicly you would gain some with a new vid at present but a p3/700 just wouldn't really do justice to a say gf4 card
Say NO to gf-MX's

lol
Have Fun
Def