Sounds like you're looking for an AGP solution, it may payoff more to get a PCI-E motherboard and video card, but it is your call. If you're going for a 6800GT it may be worth it, but the 7800GS's probably aren't very cost efficient still.
ok cool. Also, I was reading up a bit, and am wondering about my psu. I have Antec's SL400, will that be enough for 7800 GS/ 6800 GT? I also have asus A8V mobo, 1 hdd, soundblaster audigy 2, 2 fans, cdrom drive and dvdrom drive.