my brother-in-law has a dated 8300GS that came with his system when he got it ~3 years ago. His general system specs are C2Q (94xx or 95xx), 8gb ddr2, and a 350 oem psu with a single 25A 12V rail. I want to give him an extra 5770 I had laying around.
For name brand psu's, like corsair, antec, thermaltake, 80+ rated and the like, no question a 25A/12V rail is plenty of power, especially for a 5770. guru3d, using a 9xx i7 core cpu and other power hungry components equipped with a 5770, only used ~255W (total system load). knowing hp systems and their components, could he rely on that 12V rail or should I get him a new psu, such as this: http://www.newegg.com/Product/Product.aspx?Item=N82E16817152040
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AMD site states 450w minimum psu for the HD 5770. Could you get away with using a 300w psu? Maybe. You should bite te bullet and get a unit rated above the manufacturer spec, though.