Hi Guy,
I'm currently in the process of switching from External raid on firewire 800 to SATA. and first indications are showing massive speed increases. The system is yet to be fully installed so I can't give you any benchmarks yet.
One word of warning for those looking for G5 internal solutions for raid drives. I bought a device to mount an extra 3 drives inside my G5 and though the unit works in principal, the drives won't power up all together without tripping the G5 supply.
You can only get all drives online by powering them up - by plugging in the power connectors one by one - hard when they are inside the case! There is a feature on the drives called staggered spin-up but I've yet to make this work - the drives are WD 4000KD's.
I see from your post you refer to "Quad" by this i guess you mean G5 Quad. If that's the case you'll need to ensure you get a SATA 2 PCI Express host card for the SATA drives, also there is this very attractive looking enclosure on
http://www.transintl.com/store/minig.cfm that will match your G5 rather nicely - the other enclosure does look rather "PC land!"
Julian