First of all, how are you going to mount and attach the Raptors?
The Powermac G5 has room for the stock drive plus one extra drive, and it has 2 SATA connectors. So you'll have to pull the stock drive, or get a SATA PCI interface card and go external. (Yes I know there are expensive brackets to mount additional drives where Apple never intended and I think they are A Bad Idea TM)
If you get a PowerMac G4 your have space for somedrives, but no SATA onboard, so a card, again.
Secondly: Why?
The reason for RAID 0 is to improve the data throughput on high-volume, sustained load, or to create a larger single volume from 2 drives. #2 is out, because youre choosing 74 Gb Raptors.
Frankly, I am not sure you'll see any perfomance increase. If you RAID your boot volume, you will have a bunch of random and intermittant access, not sustained access. Plus you'll have a much better chance of catastrophic failure, where a problem on any one drive will take out your entire system.
www.storagereview.com has some good but lenthy articles on the pros and cons of RAID -- and their conclusion is that it makes no sense for typical desktop usage, it should be reserved for high volume requirements like video capture data and servers. If you were doing video, a 158 Gb RAID volume would be a poor investment...
How do you do it?
You'll boot from the OS DVD (unless you have gone external with the Raptors and have retained the stock boot drive), choose Disk Utility, go to the RAID tab, select your two drives and choose Stripe. That's it.