If you have a P35, you can't run SLI. Any/all Intel boards 965 and higher that have more then one PCIe slot only support CF, not SLI. You would also need to make sure that you have a PSU that can power two of those cards. Running two 9800GT's in SLI would be a good idea, but if you have the P35, that option doesn't exist.
What is your budget like? What motherboard do you have? What games do you like to play? What resolution do you play at? What PSU do you have? As always, the devil is in the details, and we need to know much more.