Yes, both the cards I linked to will work with your motherboard. Your motherboard will take any PCI-Express X16 video card. If you are going to get the 6600 GT, then MAKE SURE IT IS PCI-E and not AGP, otherwise you will have to return it and that will be a big hassle.
All the X800XL's are PCI-E except for a couple but those are in the $360-$390 range anyway so not really worth it, unless you want the All-In-Wonder one with the remote control and all that crazy stuff.