You do not want to compare the desktop and mobile 200 and 300 lines from nVidia. They're all based on the GT21x core anyways. The G/GT 300 line on the desktop only exists are rebrands for OEMs.The new graphics on the 13" MacBook look to be three times faster than the old 9400M.
The "GeForce 320M" appears to be based upon the GT315 desktop product line because it has 48 processing cores, rather than the desktop GT320 product which has 72 processing cores.
More interestingly, it is integrated. What this means is that NVIDIA have created a new, previously unheralded, chipset to replace the 9400M (and no, this isn't Ion 2 under a different name) with three times as much graphics capability. The 9400M could do 54 GFLOPS of computation, this new chipset could do over 150 GFLOPS depending on operating clock.
The "GeForce 330M" does appear to be based upon the desktop GT330 or even the GT340. I suspect it has 96 processing cores and can do around 300 GFLOPS of computation.
The Core 2 Duos in the 13" MacBook are a let down compared to getting Core i3s or lower-end Core i5s.
There has been too long of a delay in updating the MacBook line - 10 months is a long time in the Intel PC world.
GT216 for the midrange 32x/330M and GT215 for the remaining line above.