You can see that the 256mb one has 400mhz less power in memory speed and basically your game experience will not be as good as you will expect it to be. Get the one with 512mb or memory.
256mb is really pushing your luck in todays games. Resolutions above 1280x1024 and with AA will suffer. But if you're on a budget and trust rebates, then go for it. Remember, $150 will be leaving your account, not $120.
If you're not planning extreme resolutions and AA, the 256mb 8800GT will do you fine. But buying a video card seems to be a long term investment for you (you still have an X850XT), so I must recommend a 512mb card. The first card peti linked to is a better long term choice than either the 256mb 8800GT or the 9600GT. You will have to buy CoD 4 separately, but you'll be getting more life out of the card and thus not have to upgrade again until later, thus saving you money in the long term while giving more performance now as well. It's the smarter choice (the 8800GT 512mb).
Don't play the waiting game. Prices always drop, performance always goes up. ATi's 4870 will be out at the end of June (according to rumours) so that may drive prices down on other products, but who really knows?