If you plan to go with another NVidia card look over the chart seen for the various newer models to get an idea. NVIDIA GeForce Family
The latest high end cards gamers generally go after will run right upto $300- easy enough for both NVidia 9800s and ATI driven 4870s. Mid range models that would still be quite adequate are running for less then $200.
For considering a look at ATI cards start off with the 2600s being mid range upto the latest 4850s for Vista. Desktop
Different brands will also play a role for selecting a good card like MSI, BFG, eVGA, Sapphire tending to be the best performers often enough. ATI is best known for image quality while most trend to NVidia for gaming performance. That is another thing to add here.