Well, couple things first. Nvidia cards don't run in crossfire. They run in SLI. And you need to make sure you have a board that supports that, ie: 650i, 680i, 750i, 780i, 790i, ect. Then, the performance of SLI vs. a single board solution varies upon the cards you have in mind. You said that two 9800 GTs would be your SLI option, so I'm guessing that a 260 or 280 would be your single card solution. But without more information it would be difficult to help you. Can you fill in the specifics, please?