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.