Just to add, I'm really pleased I didn't give in and buy that dual core 2.0Ghz G5 PowerMac last year.
Sure, I'd appreciate the upgradability and stuff, but a fully decked out Mini Duo comes in only around AU$1500, which in contrast to a base line G5 DC 2.0 at AU$2999 is *extremely* good value for money.
I appreciate the extra core too, since I have actually got quite addicted to multi processing a couple years back and since then have been shelling good money for dual socket motherboards and ECC memory modules.. with the mini, never more
Either way I expect the pricing of the Mini to drop during that next Apple even sometime in May or June where they will supposedly unveil the iBook replacement (if not then Apple is ripping us off, because Intel has just cut their costs for manufacturing the Mini). What I expect to see then would probably be the current Core Duo 1.67Ghz Mini dropped to the current Core Solo Mini pricing, and an introduction of a new 1.83Ghz model, and with similar speed bumps for the iMacs and MBP as well.
That's my guess, no insider sources
Of course Apple may really prove to be gay and just pocket the extra cash as pure gravy, who's to say?
Ultimately depends on your needs, but unless something really really revolutionary comes out in Leopard, even the current Mini Duo at 1.67Ghz has a *lot* of life in it and will definitely meet most needs for the next few years, IMO.