I'm on board with it being way cheaper just to buy a box these days.
The last system I picked up was a refurb Dell Studio with a i7-2600, 1.5TB 6GB/s HD and 8GB of RAM (2 x 4GB, so two slots left open) for $600. I mean, a 2600 with a decent board is going to run you 400-450, for only 150-200 more, you get an entire setup with a new DVD burner, case, power supply (mine had a 600w) and a copy of W7HP-64.
You could just add a SSD for your OS and whatever video card you want and have a great system.
Right now on the outlet I see a XPS 8300 with a i7-2600 with 2gb of ram and 2x500GB hd's with a 1GB 5770 for $609. A 2x4GB kit would run you like $60. There's also another one with 16GB of RAM, a single 500GB drive and a GTX 560 for $829.
If you do the quick math on that using average prices, 300 (i7-2600) + 125 (mobo) + 90 (16GB DDR3) + 230 (gtx560), it starts to look like a real bargain. You lose over locking ability, I suppose, but I think that's becoming less and less important these days.