I think it's because you forgot "When I need to buy something."
I don't care too much where products are in their cycle. If I need to buy something, I look at the market, might wait a week or so if an announcement is near, then purchase.
I think either the intel iMac or low end G5 will be fine for your needs. DV footage is particularly taxing, even doing multi-cam in FCP. If you're concerned about future-proofing everything, I would go with the tower. PCI-express allows for tons of breakout boxes for use in live production. You would need that kind of stuff if you ever move up from DV.
I've very close to purchasing the Digitech GNX300 workstation for use with Garageband, but I'm coming up short on reviews/comments of using the two things together. Does anyone own the GNX3000 or have experience with it? I'm making a trip to another guitar shop soon, but any comments are very welcome. Thanks.