The video card comes with a heatsink/fan combo already. If you're talking about a system fan, do you have any additional slots to add one? Oh you know what? Get a PCI exhaust blower for that video card too. THat thing is the best kept secret. It will help with the cooling:
This is one of the better one's I've seen.
AOC FC-2002-B Ball Bearing System Blowers - Retail
FOr video card:
AOpen 91.05210.45H Geforce 6800GT 256MB 256-bit GDDR3 PCI Express x16 Video Card - Retail
Look at the wonderful reviews for that card.
$263 after rebate, this card ROCKS. THis will do very nicely with your system. I'm even thinking about buying that one. So please do...The card runs pretty warm like all 6800GTs which is why you should also by that PCI exhaust blower to compliment it.