From your original post:
RAM, Case, Cooler, and CPU are all gravy.
I'm not sure what "moderate gaming" is, but if its in the budget I'd look at more powerful GPUs.
Either the GTX560 Night Fox linked, or even juicier, check the Graphics Performance List
for a rough hierarchy of video cards.
GTX 460 is #19, GTX 560 (non Ti) is on-par with an HD 6870 @ #11.
750w power supply might be a little unnecessary.
I would check power consumption figures once you decide on a GPU, then decide if you want to pay for possible SLi power in the future now.
That being said, the Corsair HX750 is an awesome PSU and if you're set on it then you'll be happy with it.
Amazon does have the ASRock Z68 Extreme4 Gen3 (here
Is it worth the premium? probably not, doesn't seem worth $75 over the Extreme3 to me, but I may be missing something.
Amazon also doesn't have the MSI Z68A-GD55 (G3) which I'd take over the Extreme3, not sure why they're slacking.
So, in my opinion, either work the Extreme4 into your budget or bite the bullet and order a motherboard from Newegg.
Curious about that, do you have a beef with Newegg or a good deal with Amazon?