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.
Buy when you need one. My last two purchases have been refurbs from the Canada refurb page and they are doing great. I was on the refurb page today and noticed another Apple pricing inconsistency. Apple prices their refurbs based on their original selling price. The image below shows how whacky this practice is: