oh about your question on AGP vs PCI express
if you plan to continue a regular update cycle (next 6-8 months) go pci express. However if your going to just mostly live and let be (update in like 2-3 years) you might as well go for the currently cheaper AGP stuff.
I can vouch that of the top of the line cards the 6800gt is a great value...
can clock that mother quite a bit past ultra speed
and AMD is awesome