If they can support ANY add in card (unlikely) it will not run any games. You need at least a 4870 to run console ports at medium settings on a typical 1080P monitor, which draws a lot of power.
Looking at a low end Dell at Best Buy, it has a 300 watt power supply, obviously a joke for any gamer. I had to upgrade mine from a 500 to a 650 (name brand Antec, higher quality than no name supply will be in Dell by far) just to run my 4890 which takes 200+ watts by itself. I was getting shutdowns during Crysis with the 500 watt Antec. My 4890 is hardly a cutting edge card anymore, it's mid range. The rest of my PC's specs are low end (Q6600, etc)
I'd say building any gaming rig today you'll need to start with a minimum 800 watt quality power supply, I wouldn't go with less.