580w PSU, but how much will it really use?

[Vini]

I'm building a NAS, that I want to be light on power.

I acquired a cheap Hiper 580w PSU (HPU-4M580-PK) that will be powering;

Asus P5N-T
2GB Crucial DDR2
ATI 4350HD
E6400 (65nm)
7x 1TB WD Green HDDs (EAVS)

By having a 580w PSU, will I be using more juice than if I had installed a 300w (say for example)... or will they theoretically use the same power at full & idle, with my 580w giving more headroom for expansion?

[watercooled]

Theoretically it shouldn't use any more than a 300w PSU but switch-mode PSUs tend to lose efficiency with low power draw so in practice it will probably draw slightly more than an equivalent 300w model. On a side note, a NAS isn't really something you want to be running off a 'cheap' PSU, I mean you could do worse then Hiper but the PSU is the last place you want to skimp really.