7850 is better than a 560ti, its more akin to a 570.
I've ordered from PConline a few times, and they have always delivered a timley manner. although the one time i needed to contact them was a chore. they don't seem to have much in the way of staff. however buying a card from sapphire, you wouldn't really need to go through the merchant for an RMA. go direct to manufacturer.
a 7850 should max out d3 easily. it will max out all but the most demanding games.
the 7850 is one of the better cards in NZ atm, cause it hasn't seen the same price hikes due top low stock levels. you get a lot for your money (unlike the 680 i paid 1000 for...)
if you are concerned about buying from pconline, here's a link to playtech: http://www.playtech.co.nz/afawcs0139235/CATID=/ID=17194/SID=730704745/productdetails.html
a little more expensive, but some of the best service i've ever encountered. guaranteed overnight delivery and always happy to help.