That really depends on a lot of things. That could be an i5 with 4x 680's or a Phenom II x4 with a 7770. You'll have to specify your budget, location, etc.
I recently bought a Radeon 7770. But it says it needs 8GB system memory. What do I do?
you need 4 gb of ram if you are using a single card,for crossfire you need 8 gb
and it will be helpful if you give your budget