Was interested alot but then.....help pls?
Your basic problem is going to be a simple one, but I need to digress first.
Your monitor size does not matter.
What matters is your monitor RESOLUTION.
Just to give you an idea here, Newegg.com currently lists 50 different monitors with a screen size of 24".
Of those monitors 36 have a recommended resolution of 1920*1080, while 14 have a recommended resolution of 1920*1200.
Just because that is the resolution the monitor says it can display, DOES NOT MEAN that it is the resolution the monitor actually displays. Cases in point, I had a Scepter 23" monitor. While the monitor said it could accept 1080p ... it's actual resolution was... 1366 x 768. I currently have a Westinghouse SK-32H240S. The monitor says it will accept 1080(i), but it's actual resolution is... 1366 x 768.
I hope this explains why telling us the monitor size... tells us absolutely nothing at all.
Now, back to your problem.
Your basic problem is two fold:
First, a 4870 is simply too weak to run the game in a high resolution in ultra-mode. The starting point for LOW Ultra-Mode in 1920*1200 is about the RadeonHD 4850. The 4870 will buy you more frames per second, but not necessarily any more detail.
Second, Anti-Aliasing is only enabled on ATi cards through the game.
You need to TURN OFF any AA settings in Catalyst Control Center.
thanks to the person that responded with some real info.. I went into an instance mission and put the fsaa to x4 all was fine, went back out of the instance and all was fixed AA wise.. not sure what that was about but it seems to of sorted itself and with no performance issues..
Glad it's worked out for you, as well.
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Well I like the UI and so forth, game looks fun if I can just get past this issue.. read on.

24" monitor does not normaly have issues in my games with turning off AA and still not seeing much jaggies.. this game though seems to not scale up even when scaling is set to match monitor/ui etc..
Now arma2 does this buisness of scaling to fake AA change, but then the FSAA option in game only just manages to make it look less jaggie here for me and then performance hit ensues...
Ill give the basics on card etc, ive done various tests and tried the old tweakCOH tool (but in all honesty that tool does no more than the game settings can do) and still jaggies..
ATI 4870 1gb card..
tried lower res in window modes as well btw, tried forcing AA with box modes and edge detect, nothing works via card..
turned off depth of field, ambient occlusion and water effects tried on disabled and low.. still jaggies with using cards AA settings..
The game would look great if those jaggies were not there, as the UI compliments the game well i think,with its vibrant style etc.. But on my 24" screen I just can not have such eyesores as the amount of jaggies i see and its quite disheartning..
Final note: Im using the latest ATI drivers 11.6 and i know there have been issues for some with these, but all that ive scowered on forums nobody says they broke the AA, but only some other issues with crashing etc..
If anyone can enlighten me on how to get AA to work right for me or if there is specific driver versions that only tend to work for this, then please to let me know. If i can help it I would rather not have to go try CO, as it looks a little inferior to coh..
thanks, and look forward to my eye strain being eased