A Seagate ST3300655SS CHEETAH 15k 300GB SAS drive is available from Penguin Express for $477.50. See:
http://www.penguinsexpress.com/product.asp?pf_id=ST3300655SS&gkw=ST3300655SS&gad=CMmggM8DEggtbKcHVS-B1xiX_r3-AyCt79BM
A similar drive (Seagate ST3300555SS) is available at DiscountTech for $395.00. See:
http://discountechnology.com/Products/SAS-Serial-Attached-SCSI
I've never dealt with Penguin Express or DiscountTech, so I can't vouch for either company's reliability, but their prices are the lowest I've seen so far for this drive.
IMHO, forget the G5, not nearly enough processing power (I have a dual 2.5GHz G5 that I plan to replace with one of the new 8 processor machines - - when 3rd party RAM becomes available at a reasonable price - - precisely because my G5 lacks sufficient processing power to use the 7GB of RAM it has.) Will make a good slave machine, no doubt.....
I will try a much less expensive SATA RAID 0 array - - with journaling turned off - - before springing for SAS (2 X 500GB SATA = $260 - $300 depending on the drives). Also, according to Apple', the SAS option precludes using Windows on the machine. You can get 8 port cards for external SAS from ATTO. One card (for $395) is limited to RAID 0, the other (for $1095) allows many varieties of RAID (e.g. 0.1.5. etc.) An external configuration for samples would appear to allow you to use Windows on the machine as well as OSX.
See:
http://www.attotech.com/sashostadaptertechnology.html