One of the monitors MUST (not should, MUST) be a DisplayPort monitor, can't get around that, they're a bit more expensive than a standard monitor. I'm running 3 19" monitors at 1440x900 each, for a total resolution of 4320x900, in DirectX 10 (9 crashes too much for me), with CCC Pack level 1 (which is low-medium, altered via a config file to produce similar graphics with a higher frame rate), and usually get 150 fps in the spawn room, 80ish fps in the mech hangars, and between 30 and 70 fps running around in game (dependant on what is happening).
Using a Core2Duo 2.53ghz, with 3gb RAm (same speed as yours I think). You should be able to run in 4800x900 with medium settings at acceptable fps, maybe even high, but I would suggest using the CCC Pack instead of the ingame graphics presets.
You might want to check if your DisplayPort monitor comes with a DisplayPort cable (mine didn't), and plan ahead, otherwise you will end up with an expensive desk ornament until you can get the cable. If you haven't heard of the CCC Pack before, there are links around the tech forum here somewhere to it, though hopefully some kind person will put the link in here.