Why crossfire?

So long as your cards have 1Gb each your not going to get any bottlenecks (apart from GTA IV and perhaps ARAM II).

Remember Crossfire isn't about doubling your VRAM, it allows both cards to render the same scene simultaneously thereby doubling (in theory) the speed at which that scene is rendered.
 
the point of crossfire as with SLI is to get more FPS Vram is nice but if you look at some of the tests done here on this site having 1GB card vs 2GB card most of the time make little to no difference on the other hand having 2 1GB cards might not 2x your fps but most of the time it will add 90-95%

last time i got new stuff was before crossfire or SLI were even around so i am not 100% about this but from what i understand SLI has to be 2 of the same card which is not true of crossfire if i was reading that chart somone posted right you can hook almost any 2 ATI cards togeather which will let you buy somthing new now say a 4850 or maybe a 4870 and crossfire it with the cards about to come out like the 5850 or 5870
 
depends a lot on the cards your talking about for example Nvida 9600GT in SLI even 2 of them i dont think would be as good as one GTX 260

and even if they where as close it would be better to get the gtx 260 so that you could maybe upgrade to SLI in the future