A standard 6600 performs slightly worse than a FX5900XT, unless you use some of the newer technology the nVidia 6xxx series introduced (especially shader-heavy stuff). However, LFS doesn't, so don't expect any improvement. A 6600GT card on the other hand is a lot faster than the 6600 and is a considerable improvement over the 5900XT. Since you can get the 6600 for free, you may want to try to overclock the 6600 to get near 6600GT speeds.
Nothing's lost by trying. If you use somewhat recent nVidia drivers you don't even have to uninstall/reinstall drivers. Just remove the 5900XT and replace it with the 6600. If it doesn't satisfy you or you can't overclock it far enough to make a real difference, take it out and put the 5900XT back.
