Theres no hard and fast rule either way, a single fastest card is the most ideal situation tho.
When the 280GTX/285GTX came out 8800GT SLI gave better performance for less than half the price - unless you tried to play at very high resolution - it did me well at 1440x and 1680x res - but when I moved to 2048x a single 260GTX with the extra VRAM was better - until games started pushing the power of it and then the only solution that made sense was 260GTX SLI - nothing else gave the right combination of price and performance... until the 5870 came out - but theres no point me doing a side grade to that... but now again the single card is better for this situation.
When the 280GTX/285GTX came out 8800GT SLI gave better performance for less than half the price - unless you tried to play at very high resolution - it did me well at 1440x and 1680x res - but when I moved to 2048x a single 260GTX with the extra VRAM was better - until games started pushing the power of it and then the only solution that made sense was 260GTX SLI - nothing else gave the right combination of price and performance... until the 5870 came out - but theres no point me doing a side grade to that... but now again the single card is better for this situation.