Maximum PC

So i'm looking to buy a gaming PC, hopefully pretty cheap. I'm looking to play mostly MMOs not skyrim/BF3 on max settings. I want this PC to last a few years but still be affordable. Since i'll mostly be playing MMOs i'm guessing I won't need amazing graphics cards or eight core processors or anything like that. I already have a moniter. Im hoping to spend 500-600. Try to keep it around there if you can please, and i'd like it to be upgradable if it's possible, as i'd rather not buy a whole new PC in a few years. This is what im basicly looking for.

8GB DDR3 ram
Decent processor (AMD fx 4100 looks good/affordable, quad core 3.6ghz?)
Decent graphics card (one that will last and play medium-high setting on most MMOs, looking atRadeon 7750 1GB)
1TB of memory
I have no clue on cooling systems or power, im guessing 500 watts is decent?

Help with cooling systems and power would be nice, a simple link to an explanation will work if you don't feel like explaining. Also when I go to build a computer windows 7 is adding around $100 to the price, why is that? I don't see why you'd buy a PC without an OS? Im only stating what i've been looking at to give you guys an idea of where I am looking. Thanks in advance.