1) Is it easy to overclock with this CPU / Board mix?
The ASUS gaming mb`s are child`s play to overclock.. not too updated on intel anymore, but my last comp had a rampage 3 gene with a 920 and it oced from 2.67 to 3.3 with a flick of a setting
2) RAM seems fairly cheap at the moment - is it worth paying a bit more for 16GB?
Go for a MB with a fast FSB and buy 8 gb of faster RAM (2000mhz and above)
3) Is that a good card? (And are the blue lines on the horizon finally fixed in COD for HD69 cards?)
If your playing on a single monitor, go for a fast single core GPU with 1/1.5gb(sorry not updated on ati anymore

) if your using 2 or more screens, go for a dual core one with 2 or more gigs of ram
4) Is 720W enough power?
i guess, but id go for a 1000w one for that rig... less strain on the psu means less heat development and possibly a longer lifetime
hope you find my oppinion to your liking