Call of Duty 4 isn't as costly as you think. Infact, I have a radeon X1650, and it runs at full settings with a frame rate of about 25, and doesn't even look as good as Xbox360. If you have a Radeon crossfire motherboard, or similar, you can use the Dual graphics card function, which will make it as good or better than the 360.
In otherwords, yes you will.
I dunno how good it is. I wish they had a universal system of numbers to tell, because the Radeon 9550 sucked.