There's alot to cover but briefly EWQL & VSL are both excellent. I use both in VEPro5 (which I'd recommend for anyone doing orchestration. One feature is that allows you to keep the all samples loaded in memory so you can close your daw or other projects without reloading again. It's also more efficient using VEPro5 than running them inside your daw.)
I'd first consider budget & goal.
Do you plan on doing orchestral work or using the libraries in the future?
There's a learning (& frustration) curve you will have to go through learning & using any of them.
For a student you're best bet might be GPO.
You won't get the sound quality of the others but it might be a quicker set up able to load more instruments with the small amount of memory you have.
4GB isn't very much.
It could be your best choice as far as letting you concentrate on your main objective which is orchestration instead of being interrupted with tweaking software/ hanging, crashing, freezing.
Then after your class you could decide to invest in the better & more expensive libraries.
GPO would be a minor loss compared to the others.
Good luck!