Unfortunately, right now really isn't the time to be looking at anything from Nvidia. They're beaten by ATI at just about every price since they discontinued the GTX 200 cards. The only Nvidia based upgrade to your card that isn't overpriced or unavailable would be the GTS 250 which performs close to the Radeon 4850 for around the same price.
If you still want to stick with Nvidia, I'd recommend going for something like this: Gigabyte GeForce GTS 250 1GB
- $119.99 ($99.99 MIR)
If you don't mind going with ATI and paying a little more, there's the Radeon 5770's which will give you a bigger boost in performance. Something like this would be good: HIS Radeon 5770 1GB