Probably game dependant. The GTX4xx seems to lend itself very well to SLI scaling- much better than most earlier cards
Metro 2033 for example (this game favours nVidia single card though)
BF:BC2 (pretty even, the game slightly favours AMD cards in single GPU)
AvP (Single card AMD, multi card nVidia)
DIRT 2 (Single card AMD, multi card nVidia)
(all at 1920x1080, GTX470 and HD5850)
And Tom's
SLI /
CF review (consecutive pages) at 2560x1600 (GTX480 and HD 5870)
Just Cause 2 (SLI scaling 71%, CF scaling 23%)
Stalker:CoP (SLI scaling 89%, CF scaling 56%)
Crysis (SLI scaling 87%, CF scaling: don't worry about it)
CoD:MW2 (SLI scaling 80%, CF scaling 79%)
DIRT 2 (SLI scaling 88%, CF scaling 62%)
And a very comprehensive
Hexus comparison ( GTX 470 and HD 5850)
The Cliff's notes version...
.....................................SLI................CF........scaling %'s
BF:BC2 (1920).............57%..............72%
BF:BC2 (2560).............70%..............75%
CrysisWarhead (1920)..87%.............42%
CrysisWarhead (2560)..-10%...........-51%
Far Cry 2 (1920)............70%.............80%
Far Cry 2 (2560)............89%.............76%
Far Cry 2 (2560, 8xAA)..90%.............67%
HAWX (1920).................82%.............51%
HAWX (2560).................95%.............87%
HAWX (2560, 8xAA).......88%.............49%
DIRT 2 (2560, 8xAA)... ..56%.............80%
Pick the bones out of that.