I'm thinking about building a new computer to replace my 3 year old one. I'm trying to decide whether to get a Radeon 5870, two 4890's, or just get another 8800GTS 320MB to put in SLI with my old one. Obviously the third option will most likely be the worst in terms of performance, but how much of a performance gain will I get with the first two? Is it worth the money? Which option would you recommend?