I'm not sure how useful this will be, but it makes interesting reading. Go to
http://www.gpureview.com/show_cards.php for a full list of features and to compare the performance of various graphics cards against each other. This site isn't ideal, but I can't find any other site to compare the performance of 1 card against 2 SLI/Crossfire cards. It works well for comparing single cards though.
Example: GeForce 8800GT vs GeForce 8600GT
http://www.gpureview.com/show_cards.php?card1=544&card2=513
EDIT: Click the Multi-GPU option under Advanced Options to compare a single card to dual cards.
2x 8600GT
Memory clock: 700 MHz
Memory bandwidth: 44.8 GB/sec
Shader operations: 75520 operations/sec
Pixel fill rate: 8640 MPixels/sec
Texture fill rate: 17280 MTexels/sec
Max power draw: 43W
Memory bus type: 64x2 (128 bit)
Texture units: 16
1x 8800GT
Memory clock: 900 MHz
Memory bandwidth: 57.6 GB/sec
Shader operations: 168000 operations/sec
Pixel fill rate: 9600 MPixels/sec
Texture fill rate: 33600 MTexels/sec
Max power draw: 105W
Memory bus type: 64x4 (256 bit)
Texture units: 64