Actually, Windows 8.1 probably is about 15% more efficient than 7. After installing classic shell I don't mind windows 8.1. Either way you'll probably upgrade to ten when it comes out. As for components, drop the DVD drive, get a 1-2TB storage drive and a 256gb or so SSD. You won't regret the SSD.
Ps. If you're looking for a cheap, effective cooler try the Cooler Master 212 Evo