New computer at Walmart (say i3 core desktop or even laptop) is only $300, gives you a great processor, more memory and ton more harddrive, but not much in video. So compare that with trying to individually upgrade components of a legacy system isn't worth it.
Personally I would just suggest (for average gaming) getting a off-the-shelf system then upgrade the PSU and GPU and your fine. If your serious about gaming, only systems over $750 or more are doable (see front page of tomshardware.com and the build it yourself systems, it shows multiple gamer systems).