New iMacs released -what is the difference between NVIDIA GeForce 9400M and NVIDIA GeForce GT 120 video cards?

I am going to make the switch to Mac and am debating on the new 24" iMac with a NVIDIA GeForce 9400M ($1499) and the step up model that has a NVIDIA GeForce GT 120 ($1799)...I am trying to understand the difference and if it is worth the $300 additional cost.|||I think that's the 9600GT card. Still pretty weak, but better than 9400M at playing games. Get it if you want to play games on your Mac.