Graphics card for dell xps 420

A few standard questions for ya:

1. Monitor resolution
2. Applications/Games you need it for
3. Power supply wattage
4. Intended budget

we should be able to help you better once we know that stuff  

Since your XPS 420 was ordered with the 9800 GT, it would have the 425W PSU. You can confirm this by removing the right-side case cover, the PSU has a sticker that states the wattage.

I have the same PC, and I upgraded to an Asus Radeon HD 6850 (which is available online for $150 - $170), keeping the stock 425W PSU.

This allows me to play COD:MW2 and COD:BO at 1920x1200 at the highest settings. COD:BO stutters occasionally, though other sites state that is due to the CPU.

You can go in for higher performance cards, but that will warrant a replacement of your PSU.