I don't know for sure about the 2550 but this is how it works on
our Dell 2450s with RedHat 6.2:
The 2450s have the hardware RAID built into the motherboard. If
you order a system without RAID it's not enabled. If you later
decide to upgrade you get a hardware key that fits into a socket
on the motherboard and a 64-MB cache memory SIMM.
When we first got these systems about a year ago they were
supplied with a compiled kernel with a closed-source driver for
the RAID. Later Dell updated the kernel and open-sourced the
RAID driver as a kernel module.
We ordered all our systems with RAID so I don't know exactly
what kernel configuration you'd get if you don't order it.
On systems that are dual-CPU capable you get a kernel built
with SMP--even if you only order a single-cpu system.
I'd get the RAID to start with as Dell's pricing on upgrades is
pricy.
