First of all MSI is a company, not an interface.
PCI is really slow for graphics. AGP is better and a lot faster, and PCI-Express is the future.
If you want to buy a 6800 Ultra, go for it, but a 6800GT is better for the money. If you get 2 of the same card, you can run them in SLI. Remember, you need the PCI-Express version of the card.