Um, I compile Mono (and associated parts) on a P2 300 machine quite happily...

Normal rules apply - more memory the better, faster disc speed, the better, no real barrier to the processor in use (x86, IA64, x64, ARM are supported as well as pile of others) or the such.

Compilation speed depends on the machine. A 486DX33 will take a lot longer than a P4 3.2GHz machine. Stands to reason. From scratch on the P2 server, mono takes about 40 minutes to do a full compile with gtk-sharp another 20 and other associated bits, around another 30 minutes. A P4 2.8GHz machine takes about 9 minutes for mono. You get the idea!