While the GF4MX is pretty widely accepted as a bit of a farce, (It is in fact an ultra souped up GF2 aimed at the low cost sector) the GF4MX found on the nforce2 IGP motherboards is in fact the best onboard video currenly available for any platform. (Or, at least the socket A platform. I'm not sure how Intels onboard stuff stacks up against it.)
No, its not a fantastic card. But it will perform well for older titles. I doubt it will run Doom3 that great, Doom3 is quite a hog. I cannot say on UT2K4...but I'd imagine with some tweaking it might perform ok.
Other things of note: The cards performance is signifigantly better if you are using dual channel ram. Buy two sticks, and it will do a lot better. This is because it uses shared system ram for its video memory, as stated above. (This is a pretty common occurance with onboard video, however.) The dual memory channels provide 128bit memory for the card, which hinders it much less than 64bit single channel. Of course it will actually use some of your system ram, so be aware of that if you do decide to use the onboard video. The onboard card has been shown to be roughly equivalent to a GF4 MX440 (an AGP card that is available as well.)
If you are planning I buying an AGP card anyway, I don't see any reason to buy a board with the nforce2 IGP. You're better off getting a board with the regular nforce2, since they are usually cheaper and only lack a feature you would not even be using. (Perhaps you are looking for a microATX board though? I don't think there are any regular nforce2's in that format. I could be wrong.)