I wonder how 2x crossfired ATI graphic cards could be compared with a single card with double capabilities? For instance, my video card (ATI 5570) has 1GB memory (128-bit). If I crossfire it with a second card, will it out-perform a card with 2GB memory which is 256-bit? Which one gives more performance?
No. You will still have 1GB usable memory, with 128 bit width.
(The same is true both for AMD cards in crossfire and Nvidia cards in SLI).
Most programs would only use the primary card, and run at the speed of that single card. The second card only comes into use when you have a game which supports crossfire or SLI. Then all the data stored in the first cards memory is also stored in the memory of the second card, and the second GPU takes over part of the calculations.
This means that you would have 2GB memory, but only as two 1GB copies of each other. Same with crossfire with 3 or 4 cards.
A single card with double the performance is almost always better.
The table below should be read with care, since the result vary from game to game, but:
Single card (say 100% base speed) ----------> 100% performance Single card (say 200% base speed) ----------> 200% performance Two cards (each card at 100% base speed) ---> 180% performance Three cards (each card at 100% base speed) ---> 250% performance ...
Etc etc. The overhead of extra cards causes diminishing returns.
The only reason to use more than one card in Crossfire or SLI is if you happen to already have multiple of those cards, or if you need absolute maximum performance. (And then you crossfire multiple high performance cards).