I wouldn't put too much stock in artificial benchmarks, but it just seems too hard to drive apple fans away from their unknown love for geekbench.
Yes, you will notice that difference in cpu speed geekbench shows, but only when doing the most intensive cpu stuff (such as if you encode high definition video all the time)
I think the major difference between the two models is if the older iMac has 9400m or mobility 4670. The 4670 is a discrete gpu, and will provide significantly better framerates for your games-- if you game.
The question as to whether you game or not also brings up the question of your daily uses for the computer, which should ultimately weigh in on your choice of purchase. If you know your needs won't be high-end for now and later, or rather you game or not, then I suggest getting the core2duo machine and use that 350 on something else. People like to quickly throw a shoulder at the core2duos for their whiz-bang fancy "i" branding, but the core2duos are still very good cpus-- in fact just as good as those i3 clarkdales in most cases, according to real-world benchmarks.