Guys, don't forget that because it's prebuilt it's probably going to have a PSU that is capable of only pushing out what it needs, not allowing room for much more such as a graphics card. So that'll probably need to be included in the budget.
That is a good point, and something we should consider. Even a pretty low-end PSU should be 350W+, enough to cover a lowish power CPU and a 7950...or could go a 7850 if you were particularly worried.
Granted the quality of the PSU might be a bit off...so it would be good to have a surge protected power board/switch.
edit: for PSU calcs I normally grab any online calculator and then add a bit extra "to be safe"