You're just barely able to play BF1942 then. Trust me, even on low settings you won't have a playable framerate. If all else fails, have the Gateway guys install a new card in the system - but this is just another reason you should build your own system or buy from a company that's cool enough to let you fiddle with the guts a bit (like the vpr Matrix line from Best Buy (of all people.)) Anyway, score yourself a GeForce 4 Ti or a Radeon 9500/9700 and have the Gateway guys install it, if you must. You'll see an improvement in all the games you play. Some extra RAM would help (I suspect they've managed to get the game to work with 256MB of RAM, but RAM is one of those things you can never have too much of.
Even an extra 128MB would help.)