Lol MW2... you dont need that much power to play that on max...
A 5770 can play the COD:MW2 demo at high settings on a 1920x1080 monitor. That you can get on a $650 budget. It looks really good on high, IMO. The requirements and prices escalate rapidly going from high to max settings so you probably will have to go closer to $1000 if maxing out the settings is a requirement.
really, an athlon II dual core and a 5770 would be fine to max out the game. Throw in a corsair vx450, a cheap motherboard and 2gb of RAM and you have a cheap buidl to max it out.
But again, thats assuming the OP doesn't need a monitor or OS.