1. get a 760 instead of 660, its much more powerful for just 50$ more, the ASUS version has better cooling but same performance
2. any Z77 motherboard will let u overclock that much easily, but u need a good air cooler like Cooler Master Hyper 212 EVO
3. 500W should be enough but to provide headroom for overclocking get the 600W version of the same PSU, its a good one
1. Any Z77 based motherboard will overclock well to conservative oc levels. 4.2-4.3 would be reasonable with a air cooler like a cm hyper212.
2. The asus might be 1% stronger, a difference that is not detectable to a user.
3. The 500w psu should be fine.
Some added thoughts:
1. Do not pay more for vengeae high heat spreaders, they do nothing but interfere with cpu coolers.
And 1600 speed is all that is useful.
2. The graphics card is more important to gaming than the cpu. Your cpu can support a very strong graphics card. Perhaps a newer GTX760 might be affordable if you get a stock unit without the expense of fancy aftermarket coolers. 500w should be ok here too.
3. With your budget, I would use a ssd for the os. It will make everything you do feel so much quicker. a 120gb ssd will hold the os and a handful of games. With 240gb, you may never need a hard drive at all. Consider deferring the hard drve until later.
Samsung and Intel would be my picks for reliability.