Upgrading graphics are dependent on card slot design, power requirement, and driver support.
If I remember correctly the XPS 420 has a PCIe x16 slot, so you should be fine with any of the current cards. A single PCIe x16 card slot has been a standard for all motherboards since the introduction of Core2.
Please check the power output of your power supply and make sure is it adequate for the card you have chosen. Power output is measured in total wattage.
Physical space within the case could limit the length of card chosen. The XPS 420 seems to be a larger case so only the longest of cards should be an issue.
Older operating systems may not have driver support for newer cards. Newer operating systems may not have driver support for older cards. Again I think this is a non-issue, if I remember correctly XPS 420 is either XP or Vista. Both operating systems are still currently supported by both nVidia and AMD.
I hope I didn't bore you, selecting a graphics card is pretty straight forward with only a few requirements. Since the card slot and driver support is basically a non-issue. The only other requirement that will rain on your parade is the power supply and physical space available. Your options for cards are huge, from what I can tell.
