cydonia,
In general everything one uses oneself, i.e. MY workstation, cant be powerful enough ... at least that's what I always felt like (proven by my curren cpu .-)
Like Boris told you, I doubt you will average more than 2% of cpu cycles used ... unless you run a virusscanner or seti all the time.
If you still run on 100Mbit your Network will see none of the potential crunching power present in the server...
At home the gateway we are running is a P3 500 Mhz with a 0.7Tb software Raid5, Skype isn't limited at 3 users 'cause it uses WKS cpu, the few faxes definitely go unnoticed and our ftp and webserver are only limited by our provider
Of it's 256Mb RAM mostly 60% are "free" for buffering, caching and stuff. Things only getting warm, see above, with some AnitVirus stuff - Connectivity (2MBit!) simply is way too slow

.
In summary a "few user" fileserver with some web- and a "bit of VoIP" will idle like crazy ...
and still cost lots of bucks in the electricity department running on 24/7...
Small calc: Your 3GHz/1GByte will easily run above 250-300 Watts (depending on what else you have installed), which runs up to something like 200kwh/month or above 2.5MEGAWatth/year ...
I am sure you get my point.
If it's only you (and like 5 more around) do what Boris told you ...go for fast workstation and save your savings account from the unnecessary utility bill .-)
Regards
Reinhold
P.S.: Of course THE ABOVE IS ALL BS if you are using the SME in a commercial & productive environment where the cost of 3h without connectivity will most likely exceed the electicity bill !!!
P.P.S.: Stuff the SME in the closet in your basement (cellar) but do not go for watercooling unless you are willing to check it on a regular basis.