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I'm considering buying this HP(link) and putting this Radeon x800 in it. The Video card says a 300w PSU or greater is needed and the HP has a 300w PSU, but im sure its not that great. So do you think it would have enough power?
Although the link to the HP site doesnt include their PSU specs, my gut feeling says it would be undepowered. I cant think of any OEM that ships extremely robust PSU's with their entry-mid-level systems. I'd have to say that it might work, but either not for very long or with reliability issues. Perhaps asking HP for their PSU specs specifically would be the best bet.
Googling the model number, it comes to a Merlot B 300w PSU (part #5188-0129). HP's replacement cost to you and me is $47.
2nd gut feeling says this PSU is probably at its limit or darn close. I dont think a high-draw graphics card would be happy with it.