Just blew away Red Hat 9 and installed Knoppix 3.4 to hdb on my old 500MHz box. Awesome. Thanks to Fabian and Kano for an excellent install script, and to richenbacherus for the info on editing /root/.knofig to get a separate home partition going.
It's not much faster starting KDE, but it is faster running KDE than RH was. (Then again, I upgraded RH 3 times, so there was bound to be lots of cruft).
The only really weird thing is that QTParted says my partition layout looks like this:
* = hidden 0.03M partitionsCode:|*| |-------------- hdb2 ---------------| |---- hdb1 ----| |-hdb3-| |*| / /home swap
Never seen anything quite like it. These two hard drives have been through multiple partitioning and OS changes, however.

Dell PowerEdge HBA330 LP PCI-E 12GBs RAID SAS Controller Adapter J7TNV
$18.95
Broadcom MegaRAID 9480-8e Fujitsu PRAID EP540e SAS RAID Card (Replac 9380-8e) 8i
$228.00
G-Technology G-SPEED Studio XL 48TB Thunderbolt 2 Storage - Housing Only
$200.00
Intel VROC Premium RAID Controller Card PCIe Add-On Module VROCPREMMOD
$265.00
OWC Mercury Elite Pro 2 Bay RAID Enclosure with USB 3.2 for Dual 3.5 inch HDDs
$87.99
Broadcom LSI 9305-16i 12Gbps SAS-3 PCIe x8 HBA 16-Ports Raid Controller
$64.99
LSI 9305-16i SATA SAS 12Gbs Controller card PCIe 3.0 IT-Mode w/SAS SATA cable US
$111.30
Inspur LSI YZCA-00424-101 Raid Card 12Gbps HBA HDD Controller 9300-8i
$16.99
Areca ARC-1882IX-24 (16i4e)24-Port RAID Controller Card w/BBU /HDD Monitor Board
$202.46
Areca ARC-1882IX-24 (16i4e)24-Port RAID Controller Card w/BBU /HDD Monitor Board
$300.00