I have 4 machines running full time:

1. P3 900Mhz Lappy
2. P3 933Mhz
3. P4 1.7 Ghz
4. P4 1.7 Ghz

My biggest hurdle is that these machines are all sneakernetted @ work so if they finish their WUs then I have connected up to my networked computer to get more work.