"What I currently do is use a Shure SM57 and feed it straight into the mic input on my mobo"

The microphone input on a computer is not designed for dynamic mics like the SM57. It's usually configured for an electret microphones.

So you're having to turn the gain way up to compensate and hence the noisy signal.

A simple way of connecting a dynamic mic to your computer is using something like -

http://www.gear4music.com/Recording-and-Computers/Alesis-Mic-Link-USB-Audio-Interface/ARC

Howard