I should be able to get a copy of Windows through my school. Possibly free maybe for a discount. Regardless my friends school gives its students a free copy of Windows 7 when they request it so I think I'm good.
I'm going to grab a couple fans for the case as well. I want this baby to be cold in the Summer. Any suggestion on those? I got all the peripherals (mouse, keyboard, monitor, speakers) already but will upgrade them later on. Anything that I may want or may be missing. I don't want to start building this thing only to realize that I don't have the X that plug into the Y.
in regards to fans, you will be ok to start off with as the 300 has a couple fitted. you may want to add one in future to to front of the case to draw air in, and i believe it may be a 140mm one, but dont quote me on that.
the other addition you may want to make, and which will allow you to overclock safely in the future is an aftermarket Processor Cooler.
for something like you are building, I would say something like the Thermaltake Frio.
This is also something you can add down the line, as if you buy the retail-boxed processor it will come with a stock intel heat sink and fan, which although adequate, doesnt give great cooling.