It boggles me how many of these same slimline requests for better graphics cards have arisen recently... Funny still how they all seem to arrive at the same answer:
If you have a power supply of 200W or greater, get an ATI HD 4650 DDR2 Low-Profile or an HD 5570 Low-Profile. If you have the weak 160W power supply, you're pretty much stuck with on-board graphics. Even these Low-Profile / Low-Power usage cards would strain that power supply.
from the specs on that model it looks like the 220W PSU is in it so you have to watch out for what graphics card you put in there. you will not be to put any high end graphics card in there. maybe some mid range but not many without blowing the power supply. and i am sure with most slimline cases you will not be able to just buy a new power supply anywhere and it will fir in that case
I would probably go for the 5570- probably the best you can fit AND power in that machine. Its not super powerful, but should run MW2 with lowered resolution and detail. A more powerful card will require a bigger PSU, which is probably hard to find for that type of case, as well as the issue with finding a low profile version of a better card.