Pretty nice, but I may be wrong(so don't quote me) but you may get some bottlenecking with that card and your processor(basically your processor won't be able to keep up).
If you want my opinion, and even if you don't I will still give it, I would say that for now, just stick with what you have, as those new NVidia cards are obscenely expensive(check
here).
You can check
here and see that really, you card is quite good, nowhere near the 8800, but that card is going to cost you upwards of $600 US.
In the end though, its your call, if you have the cash and are willing to, go nuts.
Edit I: Keep in mind that there are other ways you can boost performance, such as slotting in more ram(you can get a gig for about $150), as well as defragging your HDD(will optimize virtual memory usage for background processes), or by upgrading to DDR2 RAM(if your mobo will support it).