I got a really bad prebuilt pc and I wanted to upgrade the graphic card, but I'm not sure what's compatible with this thing, help is really appreciated.

My pc : http://support.hp.com/za-en/document/c04365792

Thats actually a very good PC, beside it having no SSD and zero information about the PSU. Also gimme a budget man.

Pretty much any graphics card but not too big and it depends on what kind of budget you are buying it with. Wether it's high end or medium. You have pretty decent good graphics card in your system currently so, i'm assuming you are going with high end?

my budget is around $200-$300
the only thing I'm worried is that the power supply won't have enough power

Id say you need a new power supply just to be safe. Ones in pre-built systems are usually pretty crap.

Graphics Card:
GTX 960 - $194.99 with mail in rebate

Power Supply:
EVGA 650W - $59.99 with mail in rebate

These 2 should be a nice upgrade that'll be able to handle all of the AAA games on medium/higher settings.

Too much power, turn it down to 450W.

GPU is fine

Too much? Yeah
However the deal on the PSU is great. Its quite a high quality PSU. Sure he could spend $30 on one, however I notice a lot of coil whine when using 960's with lower rated PSU's.

*cough*
Depends on many factors like build quality of the GPU and PSU (Vendor etc.). You can't really predict coil whine. This is more than fine, I mean you can power a 980ti with 450W.
He can save 20$ on this one and invest it for a 380X

Or go with a EVGA 500B and a EVGA GTX 970.