Zero - yes you can put in a PCI-e. (if you really want some first-hand advice about the x800xl just do a search and see what bedubbya has to say. his experiance was/is not good.)
Samlee- I was a little quick on the key strokes. I meant to put the 6800gt.
Personally, i'm an ATI fan. Nvidia is the best choice right now because of the PS 3.0. the x800xl is about as much as a 6800gt so why would you want to purchase a new card that is already behind on technology when you could get the newest technology for the same price?
The x800xl does beat the 6800gt in some benchmarks, while the 6800gt beats the x800xl in other benchmarks. neither card has a big advantage here.
Zero- My best advice for purchasing a new graphics card.
1. Do your home work. Lots of research and lots of questions will ensure you get what you want.
2. Don't rush into the purchase. Be patient, it could save you some cash.
3. Decide how important graphics quality is to you. For me, graphics are everything. I upgrade quite often and i don't mind laying down 500-600 bucks for a new card.
4. Finally, buy from a dealer with a good reputation. If you need to return the card for any reason, you don't want to get "the run around".