I played Call of duty modern warfare 2, and i did not face any fame problem.
my specification is below
Core 2 duo E7500 @ 2.93GHz
RAM 3GB DDR2 800 bus
Saphaire HD 4850 TOXIC 512MB
Samsung 250GB and 160GB HDD
Samsung 21.5" full hd(1920*1080) monitor
I played the game in Extra High details in 1080p and i did not face any frame problem. I played the game in both XP and Win7 64bit OS
To play this game in 1080p u must have 2GB ram in xp and 3gb in win7.
May be your graphics card driver is older.For this you faced frame problem.
You always have to confirm that you intstalled latest version of driver before playing new game.
And always download driver from AMD web site.
you can download from the link below in order to take advantage of AMD GPUs as it consists of OpenCl beta version.
I have the latest driver 9.11 but it caused problem.
after installing a modified drivers from guru3d it solved the problem.