Honestly.. that GTX580 will run like a charm. I'm with hunter what do you plan on doing that would require 2 of them?
I got a old GTX275 and use it for multiboxing wow, and it plays the new Dead Space 2 like a charm.
Personally, I like dual 18-22" monitors for a few reason.
There pretty cheap compared to the larger monitors (over 22" you start seeing a large increase of $ per inch)
Also if you playing a full screen game like say Deadspace, DCUniverse, ect..... you have that extra screen that you can look info up, read walkthroughs, cheats, watch a movie, ect....
On dual monitors my favorite thing in WOW is to read up on bosses while i'm tanking,
My wife has a laptop and uses a external monitor to watch movies while she heals... (sounds weird, but she heals better when she distracted i have no idea why)
Just my 2 cents, but with full screen games it's nice to have that second monitor that can use for a variety of reason vs alt tabing out of a game. (and if its a mmo you can't exactly pause in a dungeon....)
On the other hand, keep in mind it's very hard to find games that natively support dual monitors. your still stuck with flight simulators ect...