I've seen people claim to be able to game with the 8400M (I know, not the one you asked about), I just have a hard time believing that it's at any acceptable kind of fps. Here's a link that compares a desktop 3450 (sorry, can't find lappy benchies) to a 8400M G:
Video Card Comparison - GPUReview.com
If it's in a notebook, I have to think it's going to be a cut down version of the card...probably outstanding for video playback, and maybe even some games, and while Source does scale very well, I just don't see it happening (but please if you get it, or find a link and prove me wrong, come back and shove it in my face, I know you probably will anyway).