Asus HD7790-DC2OC-1GD5 Power supply

500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended.
600W power supply (or greater) with two 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology in dual mode.

Minimum recommended system power supply wattage is based on the specific graphics card and the typical power requirements of other system components. Your system may require more or less power. OEM and other pre-assembled PCs may have different power requirements.

Thanks for Skyrim. Well at first i really thought it was some kind of hidden key :D

It's usually described on a retailer site, you could look around where it's sold on the internet

IT all depends on the end total of what all you have plugged into the pc, but if your only using 1 highend 500W should be enough.

I personally went the safe route with mine back when 4890 was the top of the line, I got a 1000W, but I never use more then 430W in total. :P

Power Consumption
up to 150W1 additional 6 pin PCIe power required
Yes

Depends on the quality of the PSU. The amount of 450w(given it's a quality PSU) is more than enough for a GPU that only has a TDP of 85w.

OEM PSU's are usually poorest possible Bronze or below it in quality. They might have 450W PSU listed, but it highly likely won't actually give that much stable.
If you're prepared to replace the PSU later on, go for it. I predict that the GPU might not be able to handle the load in longer run.

It all depends on your full hardware not just your GPU. CPU+GPU+HDD's + other random stuff you have plugged inside your case to your PSU. And it depends what PSU it is. If it's a real no-name one then it just won't handle all your hardware anyway. Give it a bit more electricity and it will cook your motherboard. If it's something from Fortron, Corsair, OCZ or similar manufacturers and is atleast 80+ in energoefectivity it should be enough. But IMO it's sometimes better to put in something more powerful than you actually need so you can be safe nothing cooks if something happens to your power line.