I've been running Seti on two machines with XP1600+ CPUs
The one is on an Epox 8KHA+ mobo and has been averaging around 3:44 per WU
The other is a Shuttle AK31 mobo and has been averaging around 3:56 per WU.
The Shuttle mobo and XP 1600+ are in my server machine which processes significantly more requests for CPU usage (IIS, FTP Server, SetiQueue, etc.) so I suspect that is why the WU times are 12 minutes higher.
Hope that information helps!