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What's the rest of your spec, cpu and monitor res?
Keeping within budget I would go for the 448 shader version of the Nvidia gtx560ti 2gb / gtx570 1.3gb. If I was to go slightly over budget then my first choice, resolution depending, would be a 570 2.5gb.
Btw...AMD 7000 cards will be released within the next month, so it depends whether or not you wish to play the waiting game.
I7 920 @4GHz
18Gb Ram
Gigabyte X58 UD5
Dell 2709W 1920x1200.
I'd go for the gtx570 1.3 / 2.5 (preferably the 2.5), though it depends how much of your budget you 'really' want to spend.
Can you run three monitors, or two monitors and one TV off a single nVidia card?
no, but if you're not gaming, you can have a cheap card to run the 3rd monitor.
If you can wait, then i'd suggest you wait for the next gen of cards. Mid 2012 latest for the full range to be out IIRC
So I could run the third monitor/tv off my GTX285 and have a GTX570 running my two monitors?
Simultaneously?
yes, but not for gaming i don't think.
Basically, the second card is just another output.
If you can, 560Ti SLI would let you game on 3 monitors, and it would be more powerful
) and I'll see how well that works 
I'm afraid I don't know that much about this either, I'm sure surveyor will confirm
Tomorrow, I'll try disabling SLI and that should mean I'll have the same setup as you (except you have a 285 instead of a 570) and I'll see how well that works
But I'm 99% certain I'm correct. The primary monitor is used for a game, the other two can be used for whatever.

That should work, but using a GTX 285 for an extra display is a tad over kill, and probably a waste of power.
A G210 would do the job.
Yeah, but I already have one...so might as well use it.
