I would suggest building it yourself, if you really are not tech savvy enough to build it yourself take it to a mom and pop shop. I personally think that it is much better to build it yourself considering the benifits.

I have read about people using C2Quad to 5box, and after trying on my 2gb C2D rig I would have to say it is much better.

I think that good enough is in the eyes of the beholder, I enjoy running at over 20fps for WoW because I play in windowed mode and alt tab a lot, but for CS:S I usually notice anything below 50fps, so it is up to you how you want to decide.