GPUs are still useless. They can be used to speed up SOME things but most work is still done by the CPU. 320M is anyway too slow to be an advantage in e.g. OpenGL. Biggest difference is in games, in other tasks the GPU just more or less sits idle and produces the picture
Well in this case the nvidia 9400m vs. the 320m is a huge difference. Basically the faster CPU doesnt mean too much because I could say that the mini would have been better off with the i3 + the intel integrated GPU which is even slower than the 9400m (makes the 9400m look like a godsend)!
IMO the GPU is an even more crucial portion of the computer than the CPU. I've owned the i7 and the core 2 duo of the 13" macbook pro and to be completely honest, in overall usage on OSX (and I do alot of demanding things using 9 spaces) I dont see a much of a difference if any at all in real world usage. I suppose if your rendering something and want to shave off 20 minutes or so then the faster CPU would be the right choice.
Also I moved from an nvidia 9400m to the 320m (mac minis) and it made a difference of night and day. 320m literally feels the same speed as the nvidia 9600m gt of the previous generation 15"/17" macbook pro! Its performance is amazingly fast.
You dont understand how slow the 9400m is compared to the 320m. Not only does the GPU just sits idle producing picture, I can play anything on the 320m (bluray mkv files of 10Gb or higher and any 1080p content) that would stutter under even plex player with the 9400m. Now I can even run 9 spaces without it showing a hint of stutter and even run more applications without messing up my workflow because the gpu isnt too weak to hinder my workflow.
I know the plex player relies on the CPU but when connecting it to an hdtv, the GPU helped the most. Because on my 23" monitor the 9400m handles bigger mkv files just fine because the gpu is driving a small monitor but when I connect the mac mini with the 9400m to my 52" sony bravia xbr6 the frame rates slow down to a crawl and stutters greatly with the lips that doesnt match the action in the movie.
I find the 320m amazing because it gives you the roughly same performance as an nvidia 9600m gt with the much cheaper price. Basically buying the 2.4GHz, 4GB RAM, nvidia 320m mac mini or 13" macbook pro is roughly getting you the last generation 15"/17" macbook pro with the nvidia 9600m gt in overall speed. Thats a great deal as the last generation mbp still cost around the $1800.