I am considering a 3 x 5870 mining rig with a 1200wat psu and a 990fx mobo, maybe a 8 core processor to also mine cpu coins.
You could do that if you want to specifically waste power and produce heat. The 5870 will burn out it's VRMs in no time mining scrypt coins.
Or you could use 3 HD7950 and the cheapest CPU you can find (Sempron X2 190 is the one I'm using on a SABERTOOTH 990FX).