The xServe is so low fuss. I have the system that Richard describes and I can run 2 Cats from it without a fibre channel router.
The extra features of the xserve that are worth noting are:
1. Fibre channel is so straight forward. One cable to connect which can run substantial distances if required.
2. Dual redundant power supplies, Cooling systems, Battery backup for cache.
I had a G5 die on me the other day and I was able to swap it out in 10 minutes because all I had to do was swap a card and install the Catalyst software. If I had a machine loaded with drives and cards this wouldn't be so simple.
I think you would find that the XServe is very cost competitive when compared to that much SCSI hardware.
cheers
Toby