If you're in need of serious raw processing power, have a look at the Mac Pros, or similar windows computers. Speaking for the Mac Pros, they can have several high-end graphics cards, including one model they support for intensive 3D work.
The GeForce 7950GX2 is a 1Gb graphics card. You can combine two of these in an Sli to effectively have 2g and 4 GPUs. I personally think this is a waste of money, particularly with DirectX 10 coming out soon. You want something that is compatible above something that has stupid amounts of memory.