http://support.asus.com/powersupply.aspx Plugin a few values and it'll do the work for you. I would also recommend trying out a few options like adding a 2nd video card just incase you want to leave some headroom for SLI/Crossfire options, and also addtional HDDs/SDDs too.
If you have both then use both. If you have only one 8 pin then use the 8 pin, with only the 8 pin please do not overclock too hight. Like over 5GHz please.
To help us help you - please provide as much information about your system and the problem as possible.
I live in Mexico, and have limited options on brands, usually discontinued PSU are found as new models here :(, and the ones that are new are so expensive to afford