First of all, congrats on ditching that crap MacBook that people call a good computer.
Honestly, I'd probably sacrifice Win7 for Win8 for the opportunity of spending another 100$ on parts. With RAM so expensive now, it's a really smart move. You could go hardcore for a GTX 780, but I don't think you need more than GTX 770 to run all the newest games at great fps and quality. Plus, the extra money from that will come handy for the possible expense of shipping in handling. I'd definitely go for an Intel processor, though.