Do NOT get the GeForce 9400, it's shared memory with system memory. Get the cash for the 4670 upgrade.

I have a 24" iMac, and a Mini, last generation. They look similar on paper, but they are most definitely NOT equal - the Mini is a lot slower than the iMac, they use more laptop parts for it, like the HD.

The Mini is a nice machine, and the new one is bumped up nicely - the two display ports is nice - but the 9400 is just not a good idea, even for running one account.