Try going down to one column at 100% width around 480 – 520px. Equal margins for left and right (0 or auto).
I'd switch to the mobile menu version around 680 – 700px.
For phones, you need to keep your links/buttons large enough for someone to tap without hitting the wrong one. Your mobile menu buttons could go 100% width, with some top/bottom padding and a bit of bottom margin to separate them and give them more "tap space".
Angela, I saw your post in the "show your website" thread. Good work on getting so much done so quickly (speaks well for RSD's ease of use). About the 20 page limit; I was thinking you could put more of your info into sections on single pages.
For example, one Orientation Information page could have 3 sections: dates/location, online, and resources. Put buttons (anchor links) near the top of the page – under a heading identifying each page would be good.
Look other things that would work together, like one "About IGS" page that could contain the general history, alumni and a list of useful links.