This is my mobo:

 

http://www.gigabyte.us/products/product-page.aspx?pid=3759#ov

 

Have I been wrong in thinking this entire time I had SATA III functionality?!?  It looks like my board might only have a native SATA II controller?!?

 

I know my current Mushkin ssd is SATA II - I knew that going into it because at the time sata III drives were realllllly pricey.

 

The 500 GB sata III Evo dropped to 250.... and I was about to pull the trigger but I wanted to be sure my board supports the higher speed.

 

Additionally, a few months ago I read something (maybe by Bob wk6d) that if another non sata III drive is plugged into that same series even the SSD will run at sata II speeds?  Is this how it works?

Yes that motherboard does support SATA 6Gb/s connections - The internal SATA 3_0, SATA 3_1 ports and the external eSata ports support it.

 

Just make sure you connect your new drive to one of those ports.