Originally Posted by wizdom
Many of the earlier Socket 754 boards were AGP. I've had a GeForce 6600 GT AGP work fine on a S754 mainboard under 64bit Linux. What bugs did originally exist, have been fixed.
However I would not recommend investing in the card until NVIDIA release an Xorg 7.1 compatible driver. Modern 64bit distributions simply do not work with the current driver.
What a load of bull most "modern distros" are not even shipping with Xorg 7.1 Gentoo does not have it unless you go out of your way to get it, Fedora the same, Ubuntu no, Debian no I image the same for most others. The vast majority of Nvidia cards work fine with the current Xorg 7.0 and when Nvidia gets around to shipping the next driver in a little while it will support Xorg 7.1 so give it a break with the FUD.
To the OP I would go with the 6800 out of your two choices listed as it has the DDR3 and 256bit memory interface. I have a 5900 I got used that I am very happy with so if you thinking the second hand route maybe consider something like it.