yes a 4850 will be able to play cod 4 maxed out, and on the w/ the brands i really look at warrentys and the cooler they have one the card, i really like power color cards because they have some nice coolers on them(not the stock reference ones) and w/ the coolers they have ramsinks on them, they also offer a 2 year warranty.
visiontek has a lifetime warranty but they are usually a lil more expensive.
the problem w/ the sapphire one ur talkin about is yes it has a aftermaket cooler but it really doesnt look that good and it doesnt have ramsinks on it
i personally would get this:
Newegg.ca - POWERCOLOR AX4850 512MD3-PPH Radeon HD 4850 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card - Desktop Graphics / Video Cards
but i would think that ur 7950 gx2 would be able to play cod4 maxed out cuz it really isnt that demanding of a game