like mentioned, try to go dualcore.
memory/ram has a lot to do with performance too.
the intel processors are way faster than the G5s. Simple as that.
If you can, get something with a core 2 duo processor.
a mac mini suffers from the 2.5" internal drive. It might not bug you, but chances are it will. The mac pro might be a bit expensive.
an iMac might be what you are looking for? Mac Rumors: Apple Mac Rumors and News You Care About
has a forum, and they have lots of good info on all macs. don't be afraid to ask there. They will most likely have benchmark tests for pretty much anything and everything.
also, check out Real World Speed Tests for Performance Minded Mac Users
they have tons of benchmarks, reviews and tests and comparisions.