Re: Upgrade from 5850: which one?
24" monitor, multi GPU is not needed at all. A single GTX 570 or 6970 would work fine, a GTX 580 would work even better. Unless you're needing to push 3D Vision on that 24" monitor because it's a 120hz, you won't need more then one of the above GPUs for games to run like melted butter.
Ci7 2600K @5Ghz/8Gb Patriot Viper II/Asus Sabertooth P67/EVGA GTX580 Black Ops/2x Patriot Torqx 128G(OS)/1Tb WD Black
Ci7 920/6Gb Corsair DDR3 1600/GA-EX58-UD3R/Radeon HD5870/750Gb Seagate ES.2(OS)/2x 2Tb WD Green
Ci7 860/4Gb Patriot Viper II/EVGA P55 SLI/Radeon HD5870/750Gb Seagate ES.2 (wife's rig)