For just a few bucks more, you can do even better.
CPU: AMD Athlon 64 3200+ Venice
Foxconn 6100K8MA-RS Socket 939 NVIDIA GeForce 6100 Micro ATX (w/ AGP if you have an AGP vid card)
ASUS A8V-MX Socket 939 VIA K8M800 Micro ATX (w/PCI-E, but I doubt you have a PCI-E card)
CPU + Foxconn mobo = $113.49 ($103.49 after MIR)
CPU + ASUS mobo = $111.49
Both mobos have onboard graphics (in case that's what you're using anyway).
The main drawback to this route is that you'd have to reinstall Windows and everything else on your hard drive (just make backups of all your data before you do it).
EDIT: These are microATX mobos because I didn't know what you needed, and most prebuilts (HP, Dell, Gateway, eMachines) use micro boards.