Minimum hardware server spec for ISPCP?

[kilburn]

Absolutely yes: 2 gb of ram should be enough to prevent swappings, and the cpu is very unlikely to be the system's bottleneck (slower cpu means slower everything, but not "non-functioning" anything). Disk IO speed is more of a dealbreaker though, so you should check this also.

Obviously, all this depends on how you set up the system (anti{spam,virus}? fastgci/fcgid or mod_php? etc.)