Is the power supply sufficient?

That is plenty of power for that system. It will work just fine.

Intel G840
http://www.amazon.co.uk/Sapphire-11199-00-20G-HD7870-GD...
Why?
http://www.tomshardware.com/reviews/gaming-cpu-review-o...

Gigabyte Z68
http://www.tigerdirect.ca/applications/SearchTools/item...

Corsair PSU
http://www.tigerdirect.ca/applications/SearchTools/item...
(see the PSU guide linked in my sig)

7750
http://www.tigerdirect.ca/applications/SearchTools/item...
Why?
http://www.tomshardware.com/reviews/gaming-graphics-car...

You are looking at probably a 350W build max. a good 450W PSU would be plenty.

Don't get the one you picked though. its only rated 460W on the 12V rail and that alone tells me its a bad brand. Look for Corsair, seasonic, OCZ, XFX, PC Power and Cooling etc to find a decent PSU. Crap ones can take your system out with them.

is this the right intel g840?
http://www.tigerdirect.ca/applications/SearchTools/item...
the one you linked is of a sapphire hd 7870 gpu.

i was also looking for a mATX board so is this one around the same or is there a better equivalent?
http://www.tigerdirect.ca/applications/SearchTools/item...

Sorry, yes that's the right CPU. An improvement over the AMD CPU for a few dollars more.

The MSI board is probably good. They used to make boards that died after a few years, but I have used them in several AMD builds over the last 3 years and they are still holding up... I try to recommend ASRock first, if possible.

Unskol is totally right about your power usage. I was just being a bit lazy there. Choose the 500W version if it saves you money, choose the 600W if you hope to make big upgrades over the next few years.