I think you could go with a quad core CPU, but it's pretty cheap for a Xeon x3210 or x3220 (3220 is the same exact chip as Q6600), they're around $280-$300. 8megs of cache, should be plenty of cpu power for what you're doing.
You should get 4gb minimum RAM since you're running sql server.
You should have a fast disk system. What exactly is the context in which you're using SQL Server? Is there an app installed somewhere that will use the database, or is it all ad-hoq queries? If it's a web application that hits the db, are only 10-15 people using that web app? Do you know how much storage you will need for the SQL server database? The storage disk used for file server should be a physically separate drive from the one holding the SQL databases (the actual data, not the install of the SQL Server application), that way the machine can run queries for SQL while acting as a file server at the same time without too much slowdown. So maybe two separate RAID arrays for this. I don't know enough about storage to recommend a config that would work for you in terms of running both RAID arrays via onboard RAID controller, or using an add-in card for one or both arrays.
I just ordered the parts for a personal machine I will use for SQL database development, for large databases (over 250gb), intensive queries. My code, execute, & verify-results loop for the db development I'm doing will be sped up greatly by having the fastest possible disk performance, so I went with four raptors in raid0 (on an add-in controller) to hold the sql db's, and using another raid array of cheap drives with onboard raid for storage, and a cheap drive I have lying around for boot/os.