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.
Nonsense, you don't need more than 1GB of ram to run a linux miner. I have one rig with 2GB, and two with 1GB and they all hash wonderfully fast. What really matters for scrypt mining is GPU ram.
This is not accurate. When the miner software uploads code to the videocard, a large amount of ram is required. The higher the
thread-concurrency the more RAM is needed, even more with some larger hand-optimized binaries out there. This is mitigated by the swap file, but considering the cost of 4GB RAM vs 2GB in the context of a $1500+ rig, it is silly to restrict someone else's machine to just 2GB even if it works for you. They may be interested in installing utility software and/or Windows 8 64bit instead