I just called up the only Apple store anywhere near here, and they only have the 8GB models in stock (except for the top tier, with 2.7ghz, 768GB SSD and 16GB ram), so that's unfortunate. I really want to get it next week, but the only way I can is if I get it with 8GB of ram. I'll have to research it some more and make sure 8GB will be fine for me.
Yeah, the 2.3/16/256 is a BTO (Build To Order) computer. Meaning it needs to be ordered online, you can't walk into a store and buy it.
Just to give you some reference, I ordered that config. from Apple.com on the 28th of June. My order is still processing and I was given a 3-4 week wait time to ship. I have a delivery date set between July 26-Aug. 1 (4-5 weeks total). I know the wait time has gone down (2-3 weeks), but I would not expect to get your machine for at least 3 weeks once you place an online order :/
(I know some resellers are getting this model in, but I don't know wait times or if their honoring the 'student discount'.)
I really think you would be OK with 8GB of RAM, but if it's going to keep you up at night and you have the cash, you might want to consider waiting 3 weeks and getting what you want. Ultimately, you're the only one who can make this choice