Well, I wouldn't go 590gtx at all because frankly, too many issues, too many potentially awful flaws(we should know in a few weeks tops if its REALLY awful or just, not great). 580gtx is a good card but, see I don't want to say sli/xfire is a waste at 1920x1080, its not, if you want high framerates, whatever you want to spend is fine. I remember only a 18 months ago people were fine with sli/xfire at 1920x1080, now its "its only for 30" screens or eyefinity" , rubbish. High performance is high performance, for huge screens or eyefinity it will likely be the difference between playable and not playable, some games will go from 20 to 40fps, workable or not playable in many games. However theres plenty of slowish games on great cards at 1920x1080 and there WILL be more, if you want uber performance, who cares.
The thing is though, I'd still not recommend the 580gtx either, go for the 570gtx, the 580gtx costs far more than the increase in performance you get. IE £255 for cheapest 570gtx, £380 for a 580gtx, thats 50% higher cost for a card thats, realistically 15% faster at best. Its just crap value, the 570gtx is one of the better priced Nvidia cards, if you were going for sli, it beats 590gtx, smashes it when both are overclocked, and costs a lot less, will run quieter, theres no downside basically.
560gtx sli is also a very good value setup, noticeably faster than a 580gtx, ok, its not as fast as 580gtx sli, but how much more performance than a single 580gtx do you want, 100% more, or 30%, 50%?
Basically the best value setups around are 6950's, 6950 xfire, and probably 560gtx/560gtx sli.
As for AMD< 6950's, more so the reference ones that almost every one unlocks into a full 6970 for the same price, are awesome, you could always try one, if you don't like it you'll have zero trouble selling it, and move to a 570gtx after.
Theres plenty of choices, the 6990 or the 590gtx are the two WORST possible choices right now, theres cheaper and faster, and quieter setups that all overclock further aswell.
