^ You should search for 5970 in crossfire. Since it's technically quadfire, the scaling isn't all that great, but on top of that, many games seem to have issues with it. A better option would be 5970 + 5870, or just go with a pair of GTX 480s lol.
Also... more memory doesn't give more performance. It just gives more headroom to preload more textures as well as for AA, but even then 2gb is a lot.. supposedly with higher resolutions (like, more than 3 monitors) you can use up all that GPU space, just not normally. But since it appears money has no value to you, then by all means go big!