Definitely not a GF6200
The x3100 is definitely not 6200 level in performance. According to
www.barefeats.com (with their new benchmarks released today), performance on Halo is fairly... abysmal. 22 fps at high quality 1280 x 800 - I played through the game on an iMac G5 at that resolution 3 years ago and it was at least as smooth as 22 fps, though that's not saying much. The iMac had a laptop GeForce fx5200, well known to be one of the weakest cards of that era.
The other comparison that I want to share is with the PowerBook G4 12" 1.5 GHz that I no longer have - also with the GeForce fx5200. I benchmarked Halo on medium quality on that machine (1024 x 768) at 19 fps, which I highly suspect was processor limited, given that the performance was worse than that on the iMac G5 with the same GPU and a much faster processor.
I also played the Doom 3 demo on the PowerBook - painfully slow, but playable (albeit painfully). Certainly no slower than the 7fps that the x3100 seems to be capable of doing.
Moral of the story: the MacBook now has a GPU that's roughly equivalent to a fairly weak three year old dedicated laptop GPU. I haven't played BF2142, but I'm guessing that you'll have a fair amount of difficulty keeping up with an x3100, sadly.