First, both Radeon and Geforce are proven products that work well across many games....so asking which is better without a context for two complete product lines is akin to saying who would win in a fight the X men or the Justice league.
Secondly, 2 cards vs one card.....again with the "better" better for what? better for your wallet? better for thier profit margin? better at heating up the house? I know the answer is better at playing games ....but at what resolution, with in what budget, paired with which CPU?
Not knowing any of this I will proceed to try to answer your questions.
The best cards on the market are:
GTX 460 1 GB
GTX 560 ti 1GB
Radeon 5850 1GB
Radeon 6950 2GB
All 4 of these modern DX 11 cards hit the sweet spot on value/performance/efficiency.
What happens when you add a second card.....at best it translates into a ~40% increase in frame rates.
Is that better? Well for most of us that are gaming on 22"monitors or HDTV's that means going from 50-60 FPS to 75-85 FPS ....which results in little to no visible improvement.
If you bought the $600 card then you probably have an extreme CPU and an eyefinity Jumbo-tron set up to watch it on.