I am pretty sure my gphx card can display 2560 x 1600 on 2 monitors at the same time, jsut it wouldn't be good at it.
To play at 1600x1200 is about the max for your card at decent frame rates, and I play at 1440x900 with all the eye candy turned on - I would have to sacrifice that eye candy if I had a bigger monitor to display it on.
In general, as you near 1900x1200 (no need to use HD terms, this is PC gaming we're talking about) you need a gphx card with more RAM. I have one with 256mb GDDR3, but a 512mb GDDR3 would prove to be better on those resolutions.
Also you can crossfire/SLi the gphx cards and run 2 of them together. This doesn't get you x2 performance, but it does the job lower systems can't - for example running any game at 1900x1200 or higher should need a multi-graphics solution.
Regarding your monitor - it has a MAX resolution of 1600x1200, also known as native resolution. It will not go higher, that is the physical capability of the monitor. You could put another monitor on your setup, if you have 2 outputs on the card, but again the max resolution is whatever the monitor is. Obviously having 2 monitors running will put more of a strain on the gphx card, so frame rates will be lower.
The 360 game is designed for the 360. Every 360 is the same as the last, apart from hard drive size, and so they can all play the game similarly. The game is designed not to go past the limits of the 360, else it would be crappy.
Every PC is different, from Monitor size to CPU speed, gphx card, RAM etc etc - this means the game has to be alot more generalized in that it will be played on a variety of systems so can be played in low-quality mode, or super high quality mode. Basically if you are experiencing low frame rates, you either need to upgrade, or turn the settings down
Hope that helps, although I need sleep