1) all the MP AMD chipsets, the dual P3 chipsets, the dual Xeon chipsets are all supported. RH 7.2 may be stretching it back a bit far. RH 7.3 is the 2.4.18 kernel, which is close enough to hand upgrade to 2.4.20 without hastle. All the hardware support is in the kernel, and all of the annoying software reliance like for something annoyingly in the darkness like SunONE will still be there.
2) Install or compile for HIGHMEM, no worries.
3) yep, this is a hastle sometimes, but very few manufacturers of these card don't produce linux drivers thanks to Linux's invasion of the server market.
4) IDE, it'll work under SCSI emulation, hands down. SCSI will just work, its just a matter of having an updated set of cd-record utils and its friends, an easy RPM after install.
5) Cool, even though there will be a lot of fun stuff.
6) Backups to ZIP? yikes... Aights, USB 2.0 sucks back in 2.4.18, 1.1 support is fine and of course all the old parrallels work. It'll just register as yet another generic scsi device.
While shopping around, take a look at the SCSI controller onboard (if you want to go that route), or the card's chipset. Luckily they advertise this kind of info instead of letting you sweat it like onboard sound and video and the like. Either post back here or www.google.com/linux
for the info, I really doubt you have much to worry about though man.