First I want to start with a warning. It's important to do this stuff right. If a server is set up correctly, it can have catastrophic consequences in the future. If you don't know what you're doing, for the sake of either your company, or the company you are doing this for, hire a professional who has done this before to do it. Doing this improperly can cause years of frustration.
Now to answer your specific question, it all comes down to how many users you will have and what kind of email volume you will have. For an SBS server with low usage and just a few users, you could get away with 8GB of RAM and 500 GB of storage. You should use good server hardware like a Dell PowerEdge server or HP Proliant. I highly recommend that you have a RAID array set up since hard drives are the most likely part to fail. I hope this information is useful, but again, if you are putting this into production and not just for a lab, please don't try this yourself.
BL Technical Services
IT Support Maryland