Khell_Sennet did about as well as I could to explain the different technologies, pluses, minuses, etc, but I thought I'd try and respond more directly to your other question:
Kikosemmek said:
Is it a worthwhile investment or should one just get a better, single card?
This is sort of a case-by-case decision. The easy part of the answer: if you already have the single best card available, but you want better performance, you can't just buy a better card. Dual-card solutions allow users to break the performance limit of individual cards.
If you already have an individual card that's capable of being used in a multi-card setup, the question occurs at time of upgrade: will I get more bang-per-buck by buying an additional card to go with my current one, or scrapping the current one altogether for a faster single card. This will vary based on the current state of the market, where we are in the various companies' upgrade cycles, supply of cards compatible with your existing card, what other pieces of hardware may need to be upgraded to support it (like the motherboard), etc.
And, if you're looking at getting something right now, and you don't have a multi-card capable setup, potentially you want to get the capability, and a single card. Then, when that no longer satisfies your gaming requirements, you shift into the prior category, but in a pre-planned way.
Final answer: it all depends.
