There are several things to keep in mind about the overall purpose of the system...
If it is a going to be used as a file server... then sticking a bunch of bus mastering PCI SATA cards (RAID or otherwise) in a G4 is just peachy, but if the unit is going to be for Audio or Video and will have a PCI Audio interface, UAD-1 Card, Powercore Card or PCI video capture card,
then building a fast HD system that produces Audio Pops or Video drop-outs because of the RAID or SATA card then all this extra speed becomes useless. In other words, there is a limit to the amount of data that can be juggled around the PCI bus, and even though there are 4 PCI card slots, this does not mean that all combinations of filling these slots will play nicely together, so you have to prioritize what is the most important cards that are going to be used for the overall functionality on the system. Some combinations that I have tried with no issues are... DA or MDD with (1) M-Audio 2496 or Motu 324/424 Card with (1 or 2) UAD-1 or PowerCore PCI cards, I have also used FW interfaces with SATA PCI cards and had no issues, I have been up many a late night when trying to combine HD SATA PCI cards with Audio or Video Capture cards and have had some real headaches.
In a DAW environment, the SSD Drives that I use from OWC connect directly to the ATA-100 bus (via an included SATA to PATA adapter)...
http://eshop.macsales.com/item/OWC/SSDMXLE120/These drives boot Mac OS 9 in about 9 to 11 seconds and I use them for my OS, Apps, and Sample Libraries... and use 500 GB 7200 PATA IDE drives for Audio Projects.
Chris, one other note is non-SSD drives (both PATA & SATA) drives differ greatly in Speed and Cache... the smaller size drives like the 80GB may have as little as 8MB or 16MB cache so they will be a lot slower than another drive in its class that is larger (like a 500GB with 32MB cache)