According to Dell, the XPS 630 uses a 750 Watt PSU, so there's no problem with power but we need to know the monitor resolution before making any concrete recomendations.
As a non-concrete guess I'll start the ball rolling with either the AMD 6850 or Nvidia GTX650Ti, they're very similar in performance and about as powerful as your CPU can drive, if you have a 1080 monitor.
It can be VERY daunting-there are many, many, many variations out there and even opting for-say- a GTX650 will show close to a dozen options.
I'm sure others will have different opinions but I'd grab a HD6850 while they're still around. Cheaper than the 650Ti it has very similar performance and is far ahead of the only slightly less expensive non Ti GTX650.
Here's some Newegg prepared earlier:
In the above review, they use a much more powerful CPU/RAM configuration but also max out the game settings. While you will NOT see those framerates at those settings, the cards will fall into the same hierachy.
Thanks for your reply coozie7, and I appreciate the links. This helps get me started, and has led me to more sources of information. It's good to discover there are gpu options to help me get a little more mileage out of my "old" machine.