Get something with 5-2ms reponse time and DVI input, after that its quality of the picture. You have a couple more options which are 16:9 or 16:10 ratio, in other words 22/24 1680x1050/1920x1200 or 21.6/22/23/24 1920x1080 and then you can decide wether you want HDMI which basically means you can hook up a PS3/Xbox 360 as long as the monitor also has audio output.
For just PC gaming I suggest a 16:10 22" which would work well with your HD4850
I've also heard good things about the E2200HD. If you're prepared to go down 1920x1080 you should maybe take a look at the Dell 2309W or 2409W, I've seen from direct E2400HD vs 2409W comparisons that the dell looked a bit better.
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