Dual 580 on a 24in monitor on a "cheap" 24 inch monitor seems like total non-sense to me.
When I went from my 19in CRT (18 viewable) to a 24in monitor, I thought I was going overboard, but it only took a few months for me to think that I could also have gone 30in. I am not saying my monitor dissapoints me, I still love it, but you get used to those things.
Now in your situation, I wouldn't go for anythings smaller than 27in.
Because, I don't see 22 to 24 as being a change worth 400$, because of the fact you have dual 580 and that anything at or below 1920 x 1200 will be overkill. Because, if you have that kind of money to spend on GPU, you shouldn't get a cheap TN monitor, the monitor is what you look at, its a window where you can see the power of your rig (that was funny). Dell makes great monitors.
I personally wouldn't consider a 27in for myself because I already have a 24in, I am using the same reasoning for you... To that you may add that most of those are not even 24in, but 23.6, which makes the difference even less noticeable.
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