(1) 475 Watts should be fine - Cavet, how much is the 12 V rail (unk). Might try locating a model number, or Part number and do a google search. Not a lover of generic PSUs - Concur with alikum's suggestion, also consider a Corsair - on sale. Baring that.
(2) Adding a addittional HDD and going to win 7 - Not much effect as HDD is anly going to add 10- 15 W ( your current configuration is probably arround 350 W under load.
(3) Recommend downloading CPUID HWMonitor and install. With it running, run Furmark, in a window, (A free download). Note the change in +5 and +12 V. +5 V MUST not drop below 4.75 V and +12 V min is 11.4 V (I personnally use 11.6 V as a Min)
If everything looks good, then when you can afford a good PSU on SALE, do it