I did a quick test, will report back when I try it on more machines.
Pentium E5300 Dual-Core 2.6 GHz (2MB cache, FSB 800MHz)
Processor info: http://en.wikipedia.org/wiki/Wolfdale_%28microprocessor%29
Stock = 2261 khash/s
4-way = 1103 khash/s (64 bit)
Pentium 4 - 3.0GHz (hyper-threading off) 1MB Cache, FSB 800MHz
Processor info: http://en.wikipedia.org/wiki/NetBurst_%28microarchitecture%29
Stock = 1024 khash/s (32 bit)
4-way = 658 khash/s (32 bit)
Pentium 4 - 2.8GHz (hyper-threading off) 1MB Cache, FSB 800MHz
Processor info: http://en.wikipedia.org/wiki/NetBurst_%28microarchitecture%29
Stock = 917 khash/s (64 bit)
4-way = 747 khash/s (64 bit)
If I didn’t know better, I would say the key is the CPU cache size. Seems all the CPU that run slower have 2 MB or less onboard cache, where as the Core i5 starts with at least 3MB of onboard CPU cache.