Thats OK, model number will do.
you have a PCI Express x16 slot for graphics upgrades. so any card you buy needs to be PCI not AGP!
The 6600gt is a fair step up performance wise from the 6150, but the 6600 is a fairly old card by todays standards and there are better available now.
that said, assuming your computer has not been upgraded, everything else on it is also the minimum specs for MMW2 so I would think twice before spending too much on a graphics card for it.
Putting the 6600gt in your computer would certainly help, if you still only have 1gb of ram in it then adding another 1gb will probably help too, but I wouldn't want to guarantee it will play MMW2 on high details settings smoothly, even then. but it should be playable.
the specs for MMW2 are here
http://www.themodernwarfare2.com/?n=35
specs for your computer
http://www.themodernwarfare2.com/?n=35