I'd like to be able to help. but I have the same Mac (minus some Ram and with slower net) and it runs smoothly still...
Why don't you try if Acquisition and Phex work, then we might get an inkling, where the problem lies.
You can also try downloading the jum build:
http://www.mager.org/LimeWire/LimeWire.html
It doesn't need an installation (as almost no OSX App really does).