An excellent article put up by Tech-Report guys, that discerns which graphics cards are the best bang for your buck, in both single GPU and multi-GPU setups.
http://techreport.com/articles.x/14686
In a nutshell, the 9600 GT and the 8800 GT are the best single GPUs, while 9600 GT SLI and HD 3850 Crossfire are the best multi-GPU setups.
http://techreport.com/articles.x/14686
In a nutshell, the 9600 GT and the 8800 GT are the best single GPUs, while 9600 GT SLI and HD 3850 Crossfire are the best multi-GPU setups.