Check out www.newegg.com
for their stocks on video cards.
If you game rarely than don't upgrade
If you game often get a Geforce 6600 128MB AGP
If you game frequently get a Geforce 6600GT or Radeon 9800Pro
If your a mediumcore gamer get a Geforce 6800LE or Radeon X800
If your a hardcore/enthusiast gamer get a new system and pair it up with a Geforce 7800GTX.