On scratch: If you set efficiency to show on your info screen and monitor it, any time CS3 gets below about 95% you can assume it's starting to scratch. If it drops to 65%, it's scratching a lot for that image --- it's the most reliable way I've found to determine I'm actually using the scratch disk for scratching...
On drives: Historically, I preferred Hitachi or IBM drives. Then a few years ago, I found Seagates to be very reliable. I had bad experiences with a few WD drives early on -- mostly heat and noise related, not failure related -- so avoided them. Recently, WD released the Green series 1 TB drive (32 MB buffer) so I bought one for a remote quad-interface box (eSATA, FW800, FW400, USB2) to port all of my images on PC disks over to Mac format disks. Bottom line was the WD was impressive! VERY fast, quiet and never got more than warm in the NON-cooled enclosure during the sustained write and subsequent reads of 500 MB blocks of data from and to the other drives. These are still a bit pricey per Gig at $279 from
www.macsales.com.
Re Seagate: Seagate just introduced a 7200.11 drive that has shown impressive increases in sustained read and write speeds over the previous version drive, the 7200.10. Both the older generation .10 and the new .11 drives use "perpendicular" technology which improves efficiency, so either of these drives is a pretty hefty step up from the 7200.9, but the .11 is second generation perpendicular with 32 MB buffer (up form 16 on the .10). Right now, the 7200.11's are pricey at $149 (macsales) for a 500, or about on par with the 1 TB drive cost/gig. However, my local Fry's just put palette loads of 7200.10 500's on the floors at $99, IMO a great deal for a lower-cost RAID array or basic data back-up...