In my opinion, anytime is a great time to buy a Mac- especially if you are using a PeeCee. 😀
That said, I would expect Apple to start revising it's current line-up of Macs using Core Duos to using Core 2 Duos within about a months timeframe. So if you want/need the latest & greatest Mac, then maybe you should wait a little while longer. 🙂
But if you really need a computer now, the 1.66GHz Mac Mini is still a great machine. 🙂