A) Why did you respond to a thread that is 6 months old?
B) The model the OP was looking at no longer exists as a new product - all of the new iMacs are ATI GPU or 9400m ONLY - Go to Apples website and check the specs on the iMacs - of which there is only a 21.5" and a 27" version now, no 24" - back when the OP posted (in July of last year) what the OP wanted was a new machine - as it doesn't exist any more as a new option, continuing this thread is rather irrelevant.
C) ATI GPU is a DEDICATED GPU with DEDICATED RAM vs the 9400m which is an integrated GPU that shares system RAM - there is a HUGE performance difference between the two.
D) Cuda cores are a nvidia specific, and yes, a cool technology BUT to take advantage of that, the code has to be custom written to utilize those features. FCS2 - which is what this post was about - AFAIK does not offer advanced support for nVidia cards supporting CUDA, which means you're back to pure processing power.
E) The GT130, which is a rebranded 9600 is older and slower than the 4850 series that was available at the time. If it was the 9800, it would be about equal speed, but as it's not, it isn't. The GT150 is the rebranded 9800 from what I recall. If you do a search on google, I'm sure you can confirm both that the gt130 was not a rebranded 9800, and also that the 4850 leaves the gt130 in the dust for GPU power.
BTW - also, AFAIK the iMacs do not have both the 9400m and a GT120/130 active at the same time (actually I don't think in the iMac that if you go the GT that it even came with the 9400m onboard) - similar to the Macbook pros that have the 9600m/9400m combo you can either use the 9400m or the 9600m, it wasn't a summed operation where both could get used for increased performance.
Basically - in the end - for his post, the GT120, the GT130 or the ATI would have been fine selections for the system he was looking at. In the iMacs available today, the ATI dedicated GPU is a far superior option over the 9400m integrated GPU.
This is not saying the GT130 isn't a wonderful chipset and wouldn't do a very good job, but saying the 4850 is not a good option is not really accurate either.