I think you are probably spending too much on the power supply . 650 - 700 watts is fine for a single GTX 580
And yes if you do have problems with the operating system you can just by a Windows 7 64 bit OEM disk and reformat the machine with that and then install from it
Well I was thinking about OC eventually and I wanted a little bit of play with the wattage.
Thanks for your help, it really put my mind at ease....well a little bit anyway lol.