If storage is your only constraint, you could grab a SATA card, or put a big ol' SATA disk in a Firewire 400/800 enclosure, either of which is likely to be faster than ATA.
The other thing you could do if you wanted a modern computer that wasn't Intel is to look into Windows RT. I've got a Surface RT and have observed that it's a lot like a G4, the main downfall is that there's no desktop hardware that is really suitable for connecting to a monitor and a number of USB devices. The 1.4GHz quad-core ARM chip and Internet Explorer 10 are pretty good at web sites, too.
The other thing you could do if you wanted a modern computer that wasn't Intel is to look into Windows RT. I've got a Surface RT and have observed that it's a lot like a G4, the main downfall is that there's no desktop hardware that is really suitable for connecting to a monitor and a number of USB devices. The 1.4GHz quad-core ARM chip and Internet Explorer 10 are pretty good at web sites, too.