What is the best PSU for my build?

I have just had issues with my 750W PSU and I'm about to contact the seller about it but I would like to know if the PSU I will be sending back is over kill or is perfect for my build as I never really knew what one I should get. I'm just going to list my specs:

Intel i5 3570K, ADATA 8GB 1600MHz GGDR3 RAM, GTX 660 Ti 3GB SC edition, Gigabyte Z77N-WIFI Mini-ITX motherboard, 1TB WD Black HDD and 120GB 840 Series from Samsung.

I currently don't overclock any of my hardware because I can play pretty much every game I'm interested in at highest settings with a playable framerate, however I will most likely start over clocking my GPU, CPU and RAM when I start to need more power. I'm not too interested in making sure it is future proof as when it is time to upgrade my hardware I will want to upgrade my PSU as well. 

Also everything is in my Lian-Li PC-Q18 case with a PSU clearance of about 14cm or 5'' in length. 

Basically I just want good efficiency for my gaming RIG I only want what will be sufficient. I would also appreciate some guidance as to how you would calculate how many WAT's you would need in your PSU for future builds? 

Thanks in advance guys, Matt.

NobblyBobbly, You did not mentioned the PSU's Brand you totally gonna look for at least 80 Plus Bronze Certified Power Supply from a good Company. Thermaltake, Cooler Master, Silverstone. those Companies i highly recommend you for grabbin' a PSU from one of them.
( If you have the Extra amount of bucks & want some improvements for the future ) you should definitely go with Cooler Master's Silent Pro 80 Plus Gold Certified Power Supply best choice for an SLI Gamer. 

550W XFX bronze 80+ will enough for your system. Possibly the 450W if you're not overclocking. Very high quality rebranded Seasonic units, and cheap.

Looks like your chassis can hold a standard ATX power supply, so it should fit.