Mate
If you are only using it for fileserving, you don't need that much grunt anyway. Unless you have a gigabit lan (unlikely). I was hapily using a Pentium 120MHz for a network of 5 PC machine.
I upgraded it 3 years ago to a Pentium III 1.0G with 2G PC133 SDRAM. I set it up with a software raid mirror on 2 x 40G Western Digital Hard drives.
I only upgraded because my boss told me to up the grunt for future expansion. Well, that paid off because it now handles 45 pc's and 2 mac's. All have email accounts, home directories, quotas, shared ibay groups, and surf the internet all day.....i'm sure that is all they do!

Good old SARG! My record uptime for this rig was 378 days! The powersupply blew, destroying my dreams of 2+ years....oh well maybe this time...
I hope this helps with your decision, I personally would use your own desktop machine as the new SME, and use the hardware for a new desktop
