The 9400M contains 16 parallel graphics cores offering 54 Gigaflops of graphics performance. This translates to, according to Jobs, a “5x faster graphics than the chips [Apple’s] been using”. The 9600M GT doubles the amount of graphics core to 32 and offers dedicated 256 / 512 MB of dedicated GDDR3 memory at the cost of only an hour of battery time (down to 4hrs from the 9400’s five). The 9600M comes in addition to the 9400M on the Pro line for a dual configuration allowing you to switch between processors depending on your needs.
Not only is this great for performance in general, but this is a also huge win for gamers as the integrated Intel chip on the MacBooks have made gaming close to non-existent, as I’m sure many would-be fans of Spore can attest to.