The most likely update to the Mini will be the CPU process (.45nm instead of the current .65nm) with perhaps a mild speed-bump and maybe a mild upgrade to the integrated GPU (to X3100).
The CPU really won't make much of a difference (the Mini will run a bit cooler, but not like you'll be overclocking it or such). The GPU upgrade will be more important, though it is still not going to set any records for video game framerates. 🙂