Roughly 2/3 of the day, I will have the system idle in sleep mode. Is this something like 150W average?
Roughly 1/6 of the day, I will be surfing the web with the system. Is this something like 250W average?
Roughly 1/6 of the day, I will be playing games and watching videos. Is this something like 500W average?
GPU manufacturers tend to overstate their requirements in the futile attempt to compensate for all the cheap PSU-shaped objects out there. Your system would run nicely on a quality 300W PSU, like one of the 80+ bronze Seasonic or FSP PSUs in that range.
However, since you play some games, it is entirely probably that you'll want to upgrade your GPU some day, possibly soon. A 380W Antec Earthwatts can handle up to a HD6850 without any difficulty. If you might want something stronger, you may want something in the 500W range, but you certainly don't need it now. The 380W Earthwatts has been my standard PSU in budget and modest gaming builds for years now.
^+1 very reasonable estimates.
8gb 1600Mhz ram
2 DVD drives
All plugged into UPS that monitors power draw.
Power draw as i type this = 117W.
When gaming = under 200W.
The HX750 is an entirely different class of PSU from the low-wattage unit needed here. To buy one for this PC would be an utter waste of money; to add insult to injury, most of the time it would be operating in a very inefficient part of its range, well under 10%.
A few years ago, I bought a Kill-A-Watt power meter for other purposes, but it lives in my computer room full time.
One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260-216, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair TX750.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU (my "standard" and easily repeatable load), it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.
At the time I bought the PSU, I was planning on CrossFiring a couple of 4870's (I have one in another computer).
Coolmax is one of the decent (relatively speaking) PSU's available here in Saudi Arabia. They are not bad if you derate them by 20% or so (600 w to 500 watt, 500 to 400. That makes them more than adequate for basic office or internet machines.
My standard, small machine PSU used to be the Corsair CX400.