I'm somehow skeptical that the new chip will make games like CS:S or TF:2 playable at relatively higher framerates. The relative 'newness' of the chip doesnt matter...what does matter is the number of pixel pipelines, shader pipelines, and GPU clock. These variables are extremely handicapped on lower-end GPUs, and the performance of the 9400M is probably very similar to the last-generation nVidia integrated mobile graphics chip.
CS:S (which predates HL2) is a very old game compared with TF2. CS:S should play very well on the new Macbook. TF2 will give a new Macbook fits, I'm betting, as it is considered a demanding game. Even if you dial back the game settings, you're still more than likely to run into issues. I'm saying this because I have both games and TF2 gives my desktop fits. CS:S is a breeze. I've an older card in my desktop (GF 7200) but that card is a dedicated card that I can play COD4 (and TF2) on. I do have to dial back the graphics settings within COD4 and TF2 to get livable framerates, but my system runs hot even then...and this is a desktop with a dedicated (yet older) vidcard.
My suggestion is to borrow CS:S or TF2 and see for yourself, since that's the only way to find out...but I doubt it'll be much different with a Macbook.