Honestly, it's up to you. You don't have to get either. I would say go one of these directions if money is an issue:
6600GT (Leadtek, BFG, MSI, Gigabyte)-these cards can overclock very well & will give you Ultra speeds, although you have less pipes & such
x800xL - excellent card for the price, can't overclock though & costs much more than 6600GT
X800pro (VIVO if you want to flash to XT PE, and the newer ones won't do it from what I've read)- Well you'll need an older one to flash it succesfully, but it's not a bad deal if you can
6800gt-if you're going PCI-e, which you are, just forget about these...massively overpriced.