5 years ago Macs were using significantly slower processors than the Core Duo (not C2D) and YouTube wasn't even around. Imagine trying to do all the things you do now on a computer that is more than a fifth of the power of the Air. Its the equivalent of trying to run Vista on a medium quality Netbook with 512MB of RAM (an actual netbook not the 11" Air).
You don't fill your drive now, but what if your school starts distributing its lectures as video files to store on your computer. Apps get larger in size, not smaller, and almost never place fewer demands on your hardware. Case in point, google the common apps you use now and see how their system requirements have grown in the past five years.
If you upgrade computers every two years you could make do with the slower 13 with 4GB ram, but because you said 4 - 5, my vote is to max everything out. When spread over the life of your Air, the additional cost amounts to a cup of coffee per month.
If you can't swing the extra just don't skimp on the RAM. The drive is actually upgradable so in two years there will probably be a bigger one available.
