That machine will be pretty sluggish for Final Cut & the CS3 apps. The 9800 pro is probably an 8X AGP card, and most likely wouldn't work, even if you managed to flash the ROM, or whatever it is you need to do to Widows video cards to make them work on a Mac.
I'd look for at least 1 ghz +, preferably dual-CPU. If you're trying to save money by going used, keep an eye out on Craigslist for a used G5. They can be had for ~1000.00, or a bit more these days.