You'll need to use XBench, as that looks like about the only benchmark application available. It's not as cool as 3DMark or AquaMark, but it'll do. They also have a result comparison site @ db.xbench.com so you can compare your system against other people with similar specs.
MacMini - Score:
1.83GHz Core 2 Duo
2G DDR2 667
320G HDD (WDC WD3200BEKT-22F3T0) - Journaled HFS+
Test 1 | Test 2
Not bad, considering the Intel Mac Mini average is 97.27. Might have something to do with me replacing the HDD and Memory myself.
*Edit* Just ran it again after disabling TimeMachine, MobileMe, and a couple of other services... slight increase in score.