I've read that the 9800gx2 will cost 449 USD. It will be 30% more powerful than the 8800 ULTRA. So in my opinion that is a pretty good deal. For those who bought the 8800 ultra when it first came out, it's still top of the line. So the 9800 will be definitely worth it.
However, I think your processor and HDD will be a bottleneck for the videocard upgrade.