First are you sure your USB tuner will work with WMC?
For storing the recordings on the NAS, use MCEBuddy to move them over once the recording is complete. While you are at it, you can set it up to strip out the commercials before it moves them.
A WMC box will run on 2GB RAM, and doesn't need more than 4.
Noise is completely up to you. My HTPC is very near silent. It makes no noise even with your ear up to the case. Pick components that are silent or near silent and you'll have no issues. Even the stock coolers that come with Intel i series processors (i3, etc.) are going to be silent in any real-world use, as long as you have your BIOS set correctly.
My main HTPC is an i3 2100 Sandy Bridge. Anything you buy now will beat the pants off it. Your i3 choice is plenty sufficient.