Tom's Hardware Guide Processors: Fast Computer On The Cheap? The Sempron 3100 On Overdrive - Socket 754 On Overdrive: A Fast Computer With The Sempron 3100 . This is a good review of a budget machine. It uses the AMD Sempron (a cripple Athlon 64).
The CPU is just $120 with a DFI LANParty nF3 250 Gb motherboard for $70 plus $70 for Corsair TwinX CMX512-4400 Ram, it is a great budget value. Moreover, you can overclock it about 25% and get the speed equivalent of an Athlon64 3200+ or Pentium 3.2GHz for much less money.