I've got 3 systems.....all with 1-1.7ghz AMD Athlons, all with 256MB+ RAM, ect...
One has a 5400rpm Maxtor, one a 7200rpm Seagate & one a 10,000rpm Quantum. The 10,000 rpm one is obviously fastest, and the 7200rpm one isn't in 2nd place by too much (it's a noticable, but not annoying speed difference), the 5400rpm drive however is painfully slow. It seems like you're spending half your time waiting for the hard drive light to go out....very discouraging.