That's interesting because according to HP that Server is certified with RHEL 5. your install should be pretty smooth.
Certification Notice for this hardware.
No Known issues with this hardware and RHEL 5
Drivers for ML110 G5 & RHEL 5 (Including Linux drivers for the storage controller)
When you say RHEL 5 do you mean 5.0, 5.1 or 5.2 ?
That is the same Storage controller that is in my HP. Debian 4.0 (Etch) installed without a hitch detecting the storage controller, NIC's etc.. Your server is also listed as bering compatible with Debian Etch, if you want to give it a shot just to check out the hardware.