Complicated situation on building/upgrading a PC

Its_Crisis

Honorable
Nov 28, 2015
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So before I start this is a very complex situation, so I'll try to explain it, building my friend a computer, the only parts we have is an Asus Strix SC r9-390 8GB edition, and I also have an old power supply, well new but not used, came with a purchase I made locally when I bought a PC, the power supply is a corsair CX600M (600wat)
http://www.corsair.com/en/cx-series-cx600m-modular-atx-power-supply-600-watt-80-plus-bronze-certified-modular-psu


Here's where things get complicated, there is a guy selling a PC locally for $350 CAD, this is in Canada so it's a good price, here is the model
Hp pavillion 500-489 link below.
http://support.hp.com/rs-en/document/c04884772

That computer has some amazing parts compared to the i5 4690 I had chosen, as well as a available PCI slot for the card, but the power supply is only 300 watts, way too weak for a Asus r9-390 8Gb
My question, is the graphics card compatible with the computer linked above, as well as this power supply, if it is compatible but the power supply isn't, can anyone help me find a good power supply at a Canadian retailer, if anyone can help me this will save me a ton of money