You may not be experiencing games the way developers intended if you are running them on low resolutions. Being able to ramp up the resolution and quality settings whilst still pumping out high frame rates can completely transform your gaming experience, so the right GPU is crucial.
In addition, since Windows Vista ships with DirectX 10 graphics, it's important to invest in a DirectX 10-ready card like the nVidia 9800GT. That way, you can enjoy the benefits of DX10 from the start. If you are a keen gamer or you're looking for a future-proof system that will last for some time you should invest in the best graphics card, as these products change regularly and a mid-range model would offer lacklustre performance within one year.