Theoretically, 2 cards=twice the power. What you got to remember with a computer is it is a big calculator, it does calculations, your processor does calculations, your video card does calculations. If you have 2 cards, you have 2 chips, you have twice the number of calculations going on (theoretically). You never get double the power with crossfire or SLI though, you get pretty close to in most cases, but never double.
If you have an old game that would run no problems on full settings at a stable 60fps on a single 4850, getting a second would be pointless, you already have it on the best, you already have it on good frames per seond.
But, you get a more modern game and a single 4850 can only handle it on medium settings at 40fps, getting a second card would mean you could be able to put it at high settings at a higher resolution and get 60 fps.
It will increases performance over one card, but you can't really look at it as 2 512mb cards = same as 1 1gb card, it doesn't work like that.
The storage on a card is video memory, not processing power, it affects how good a card is, generally out of 2 cards in the same graphics card model, a 1gb card will be better than a 512mb, but it isn't only showing how good it is. If you were to use that logic, my 2 8600GTs that I have in my system make 1GB, those together aren't better than a single 512mb 4850, the 4850 blows it out of the water.
If you had 2 512mb cards that are of the same model as a single 1gb card, I would say 9 times out of 10 the 2 512mb cards will be better because there are 2 chips processing data about what to put on screen
