if ur PSU is good, branded reliable 450 W, u have extra juice for extra hard disk.
if it is not branded, how can u calculate 450 W.
but, if u have not Overclock any of ur components in ur pc, even 300 W PSU can handle your pc with ease. so i think 450 W PSU is enough even for not branded PSU unless, ur PSU is extremely cheap and bad one.
I have a problem with using psu calculators.
Not that they are wrong, but that we can't accurately select the inputs.
What factor do you use for capacitor aging?
What cpu utilization do you use?
How do you know your overclock speed before you get the chip?
How do you determine if a fan is low, medium, high performance?
What percent of max power should you be running at?
My point is, that psu calculators are more accurate than our ability to select the correct input.
A good old fashioned rule of thumb is just as good.
A 200-300w psu will run any pc with a normal complement of drives, fans, and a cpu, exclusive of discrete graphics.
The differences, plus and minor do not have an impact on the final result.
It is the graphics card/s that really determine the psu you need.
A pci-e slot by itself can deliver up to 75w.
A 6 pin pci-e power connector can also deliver 75w.
If your graphics card needs a 8 pin connector, that is 150w.
So, for a simple minded calculation, just add 300w plus the watts required for your graphics cards and you will get a reasonable size for your psu.
For example, a GTX560ti needs two 6 pin pci-e power leads, that is 150w, plus the 75w slot, and the base amount of 200-300w gives you a psu requirement of 425w-525w.
Also realize that a psu will operate most efficiently in the middle third of it's range.
It will also be quieter if the cooling fan does not need to spin up at maximum load.
Since a psu will not use any more power than it needs, it is not wrong to overprovision to the next size or two larger.