Couple of things:
There are only room for 2 hard drives in a G5 tower (barring expensive and/or b#tt-ugly mods), so by installing 2 x 74 Gb Raptors, you'll drop your total space to about 140 Gb.
The idea of using the 10,000 RPM drive as a boot drive/scratch drive and separating your data to a different drive is good, and has been confirmed here by a number of members here who use this in a production environment.
The concept however of using a RAID 0 (striped) array for a boot volume isn't very robust. First, your risk of data loss is multiplied, because any failure on any one drive will take out all data on both drives (that's how RAID 0 works).
Second, setting up a RAID involves a CPU overhead while the distribution of the data is calculated. The net result is that a RAID 0 system does not benefit typical desktop use, and is actually slower in many types of operations. RAID shows a speed gain only under heavy, sustained use such as on a multi-user server or in capturing large video files.
http://www.storagereview.com/php/cms/cms.php?loc=news_content&id=970&start=6&range=10 for more information.
From Anandtech.com:
http://www.anandtech.com/storage/showdoc.html?i=2101 "If you haven't gotten the hint by now, we'll spell it out for you: there is no place, and no need for a RAID-0 array on a desktop computer. The real world performance increases are negligible at best and the reduction in reliability, thanks to a halving of the mean time between failure, makes RAID-0 far from worth it on the desktop."
You would be better off with a Raptor as the boot drive, and one of the latest generation of 250 - 400 Gb SATA drives, like a Maxtor DiamondMax 10 (which is the same mechanism as a MaxLine III) with 16 Mb cache or the latest Seagate or Hitachi 7K400, for your data.
Thanks
Trevor
CanadaRAM.com