pci-e 3.0 work with 2.0 slots just fine. During Beta, my OCed 6850 pretty much maxed out DIII at 1080p with no slow downs. If you dont like to OC, than a 6870 will be great, and if you can afford it, 7850 is even better. Both will max DIII easily. If you want an nvidia card, go for a 560 GTX or a 560ti GTX.
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 only want to max out D3 and don't want to spend a lot of cash, you should look into GTX 460. It will easily max out Diablo 3, with 70+ fps. My GTX 560 Ti on public beta averaged at 110-120 fps. Your current card should play it maxed out at 40 fps atm, and unless you want to play at 60+, it's no real reason to upgrade.