Resolution up to 19xx=16xx 570/580.
Anything over, dual or triple monitors 590. The 590 is designed to do just that (inc surround vision or whatever Nvidia call it) and using it for anything less is a waste of time and money.
At 1080p the 570 is the sweet spot. The 580 is obviously faster but you need to take into account the importance of Vsync, which will cap you at 60 fps any way. The 580 isn't fast enough for the resolutions above 1080p so is a bit of a white elephant given that it can't really do much more than a 570 at 1080p.
I know you have a budget of £600 but that doesn't mean you should burn it up just 'because'.
Weigh up what you need, and then what you want. If you want to dump £600 then do so, but just know that other than your benchmark bragging scores you will probably have wasted a ton load of money that could have been invested elsewhere.
Also (forgive me for post skipping as I'm pretty busy today) Crossfire, IMO is absolutely terrible. Been there, seen it, done it. It is, apparently, why the 6990 is quite far off of the 590. They should be about evenly matched but once again Nvidia's drivers are better as they have always been.
SLI has its caveats also. For example my pal is running a 460 2win and in Left 4 dead 2 the sky is often purple and he suffers the odd CTD.
A single GPU is always the better option if you like, want to sit down and enjoy your PC. If you want to spend your days beating out benchmarks then by all means go ahead
I'd never suggest 2x cards only cos, and depending on, whether the 'asker' is prepared to not necessarily expecting 2x the performance over the single card, be a bit disappointed occassionally when certain games don't like dual cards, sometimes have to mess about with drivers, double the heat for your system.
^ Wot he said.
