I'd strongly recommend taking a look at Apple's refurbished gear. My last 7 or 8 Macs have been refurbs, and I only had one minor problem with one of them, which was fixed and returned inside of three days. For video editing, I'd lean towards a 20" iMac, which can be had for $999 at the moment. The MacMini could work for editing I guess, but I'd strongly suggest running an external FireWire drive, possibly even RAID, for your media. Also, dropping a 2GB stick of RAM in it wouldn't be a bad idea.

I have a MacMini hooked up to my home theatre projector, which does a great job as a HTPC, serving up iTunes to everyone in the house, serving TV/Movies to an Apple TV and my roommates PC's, and running bit torrent. It never hiccups, but the internal drive is on the slow side. I keep all my media files on an external FW drive.

I'd stay away from the G5's, just because they're several generations old now. I have a CoreDuo iMac 20" that's just 2 years old now that I use as my primary machine, for GuitarRig, and have even used it for Final Cut Pro. For the first time ever I haven't been able to come up with any good reasons why I would like to upgrade to a newer machine now that I've hit the two year mark.

Cheers