I'm with snapshot on this one - and the older the os the better to minimise overheads - 98SE always ran F@H standard WU's faster that XP did. But it looks ver5 dosen't surport 98 anymore..... and Ver4 will not give you the newer, larger, ram hungry, bonus pointed WU's.
Windows NT or 2000 with the ver5 or beta ver6 client.
Single core - single client
Single core with HT - two clients (HT on) or one client (HT off)
two cores - two clients
Edit: if below 800fsb P4 not worth it for a dedicated folding PC
- 80 to 120ppd for lots of £'s of electric