If you want to mine with GPU, its better chose to mine some scrypt coins, because you wouldn't get any money mining BTC with GPU.

And yes, the GPU is the most important factor, however there are some minimal requirements as well, which depends on how many cards you want to run.
Make sure you have enough PCI-e slot on your MB, and for a 4card setup you would need 4GB of RAM as well under ubuntu.
Don't be cheap on PSU.