Remember, RAID is NOT a backup solution. And from what I've been reading, software RAID is the way to go these days performance wise; something about how CPUs are so fast and what not. Me? I still run hardware RAID on our server on a Promise FastTrack. It ain't broken yet and I'll be damned if I'm going to change it.
Spend extra money on GOOD DRIVES. Seriously. Don't be cheap. I can't begin to tell you how many SATA and IDE drives I'd had fail over the years. Ok, only one SATA drive. But it was < 1 year old. Do NOT buy run of the mill consumer drives. Pony up extra dough for bigger cache and faster rotation. Personally, I'm partial to SCSI drives, probably because I've had the same drives running for like...7 years or something. Maybe more. Serial Attached SCSI is the new "in" thing. I personally wouldn't buy anything less than 10,000 RPM, 8MB cache. Also, there seems to be speculation that the larger drives have a higher failure rate; I would think carefully about how much storage you really need.
Most of the time, the CPU is not a bottleneck in a SOHO/SMB situation. I/O and network throughput, on the other hand, can be. Our server at work was memory bound; slapped in another gig, adjusted the page file and it's much faster. At peak usage it sees maybe 3 or 4% CPU utilization. I would also probably spring for a motherboard that accepts ECC RAM. ECC = Error Correcting Code. Basically the RAM checks to make sure the bits are as they should be, and mitigates the possibility of data corruption.
Also, if you're thinking about a virtual server, per the posts above, look at Virtual Box from SUN Microsystems. IIRC, it outperforms VMWare, and is generally considered superior, though that might have just been SUN fanboys ranting
