Average hashes to find a block within 10 minutes at difficulty 1: ~7 Mhash (1*2^32/600), though we are talking about an 100% coverage, whereas it's safe to assume in brute force that with a 50% coverage you've a good probability of hit a valid result already, bringing this number to 3.5 Mhash. A Pentium III Coppermine would provide about 0.4 Mhash/s, 240 Mhash/10 minutes.

As of a set of hashrate tests, even a MIPS from an old Asus router (WL-500G) would provide enough hashrate to beat the 1 block/10 minutes cadence at difficulty 1.
( https://en.bitcoin.it/wiki/Non-specialized_hardware_comparison )