You are running this on OSX Lion or a Windows partition?
Ive tested Dolphin today on my 2008 MacBook (2.53Ghz Core 2 Duo, 4GB RAM) running OSX Lion and I also tested it with my MacMini which I believe is the same as yours (2.4 Ghz Core 2, 2GB RAM) running Snow Leopard...
Neither of which can run Wii games effectively for me at least. Mario Kart Wii pulls in around 10-15fps, Sonic Colors is playable, NSMB is fine up until the desert level where it runs at 5-6fps and it feels like super slow motion, and Donkey Kong Country jumps wildly from 25fps down to 10fps and the textures are messed up. GameCube games I would deem 'ok' as most are playable in the ones Ive tested. This is in a small window and not going full screen or HD.
Personally I have yet to see Dolphin playable with most games. Oddly, mileage varies for some people. Some games run ok, but from my testing and those of friends we don't recommend it to anyone as it seems hit-n-miss. Would love to know what is going on, if there is a graphics card issue or something.