I built a friend for a computer, and here are the parts we got (2 months ago, off of Newegg. $640.92 including tax and shipping at the time and for a whole rig. It's pretty decent but it's not gonna set records either (: and the videocard could easily be replaced with something better in a year or whatever).
ASUS M4A77T/USB3 AM3 ATX Motherboard ($80)
Western Digital Cavial Black 1 TB 7200 RPM SATA Drive ($90)
AMD Athelon II X4 635 Propus 2.9 GHz 95W Quad-Core CPU ($100)
G. Skill 4 GB (2x 2 GB) DDR-1333 RAM ($95)
XFX Radeon HD 5670 Video card ($110)
Antec Three Hundred Illusion Black Steel ATX Mid Tower case ($55)
LITE-ON CD/DVD Burner ($20)
Corsair 650W Power Supply ($90)
If you ditch the hard drives, the DVD drive and the power supply, and get a few things on sale on Black Friday, I'd say that would run ~$350-400.
4 GB is good enough and the mobo has four slots so you can upgrade later on when you have the money (if you need to. Right now 4 GB is enough, in a few years it might not be).
The CPU is somewhat weak (lacks an L3 cache). But it's cheap for a quad core and I imagine it is even cheaper now. The mobo does support AMD's latest hexacore CPUs so in a few years you could drop in a bargain-bin AM3 hexacore CPU and call it a day. (AMD is dropping support for AM3 next year once they release their 'bulldozer' line. But any CPUs released for the next few months will still be on AM3).
Videocard isn't amazing but helluva lot better than what your labtop had, probably. (: Again, in a year or two you could throw in the Radeon HD 5850s which run most games right now on High to Very High, and by then it would probably be $100 or something like that.