Hi
Well i dunno about 120Gb on an Athlon 800, but my box,
also an Athlon 800, swallows 80 Gb without a problem.
How does your bios detects this drive, check that first,
maybe need to upgrade your bios orr
You can bypass your bios, by feeding the correct
drive geometry to your kernel.
So when the redhat installer boots type:
linux hdX=cyl,head,sect (or F2 and then linux etc etc)
read;
http://www.tldp.org/HOWTO/BootPrompt-HOWTO-7.html#ss7.1
so linux hda=3649,255,63 says 30Gb disk primary disk, primary
controller.
And some other options with newer kernels and Athlons in general
linux noapic acpi=off mem=nopentium
might come in handy
Good luck