Here's a short update on the latest with my SATA woes.
First, a correction: The distribution I originally had working was Red Hat Linux 9. After trying many distributions, some with interesting failure modes (e.g. Debian Sarge), I finally got around to Fedora Core 1 (I had to download it this afternoon!). This installs kernel 2.4.22, and it not only recognized my two SATA disks, but the existing LVM setup, too! Under the old installation (I don't recall the kernel version at the moment), I was getting disk-cache reads of ~87 MB/s and disk reads of 1.39 MB/s (as reported by hdparm -tT /dev/hde). Yes, appallingly slow, but it worked. With FC1, I get disk-cache reads of 1500 MB/s (!) and disk reads of ~57 MB/s.
After that, I had the job of getting the software RAID 1 working again, as one of the drives got out of sync. I eventually figured out I needed to use raidhotremove/raidhotadd (with no man page installed!). raidhotremove /dev/md0 /dev/hdg said the drive had already been removed, and raidhotadd /dev/md0 /dev/hdg got everything going again. W00t! Now it's happily resyncing at around 10 MB/s (est. 250 mins to completion) according to cat /proc/mdstat.
Maybe I should mention that the on-board SATA controller had been disabled by a jumper. :-P Yet another hurdle.
I'll check it tomorrow and run some tests.