for PCIe, X800XL is the sweet spot for performance right now.
On pricewatch today
x800xl pcie $273
6800gt pcie $366 (almost $100 more)
In this case I dont think 6800GT justified $100 more price increase for what it offers over x800xl.
Now 6800Ultra PCIe is $485 (insane price). This is plain ridiculous given this card is about 10 months old and MSRP of $499.
Given that new generation of cards are about to come out, putting down $400+ on a new videocard today is either a waste of money, or can be OK if you have money to waste. Think it over.
Start Review Here
NOTE: If you have DFI S754 mobo (ie. AGP), then like has been recommended a Leadtek $290 6800GT is the way to go.