I think the VR 150GB is or would be good safe bet for boot drive, but depends on your work, size of files, amount of memory, and whether you have your scratch volume on a dedicated drive or not, or on stripped RAID.
Number of things to look at to be sure your system is configured for best performance using CS3 (and soon CS4).
You might want to continue to boot from the 640GB and use the VR for dedicated scratch. Or buy 2-3 and stripe them into one volume as scratch.
These two give a good start, though you may already have read them before.
http://www.adobe.com/go/kb401089
http://homepage.mac.com/boots911/.Public/PhotoshopAccelerationBasics2.4W.pdf
QB is now 4.x and I know its been improved, not sure how accurate or compatible it is or if there is a free or not update to latest.
RAID isn't always the best for boot drive.
OWC $199 "pending"
http://eshop.macsales.com/item/Western%20Digital/WD1500HLFS/
DW can fix errors, but depending on how you installed or copied your system, I usually just clone (SD) and then run DW, then do Safe Boot, and then normal boot. But I keep data and media files off the boot drive.
It might seem like you want to let CS3 use your boot drive as default primary scratch location, but you don't. You might want a partition on the 640GB, or on a fast but dedicated drive just for scratch (and that gets erased as needed).
The difference between the 640 and 10K VR may not be as huge as it was from older drive though.
Take a look at Tom's hardware I posted, and there are other reviews (but no one had or used the 150GB).
One person has one in his G5 Quad (300GB) and another in PCI-X 2.7DP. The early G5 probably has some limitation, but without seeing numbers or knowing more... just guesses.
http://techreport.com/articles.x/14583
http://www.anandtech.com/weblog/showpost.aspx?i=432
PCMark05 requires both high throughput and quick access times, and apparently, fast drives do not always necessarily dominate this benchmark as well. Both Hitachi and Seagate do very well, but the drives do not overtake the Fujitsu 15,000 RPM drives—those seem to be the better choice for workstation-type PCs that have to handle lots of applications, while Hitachi and Seagate have probably optimized their firmware for server-type workloads.
http://www.tomshardware.com/reviews/HDD-SATA-VelociRaptor,1914.html