What PSU and GPU do I need for this PC?

I got this bought for me from my parents: http://www.ebay.co.uk/itm/Lenovo-H500S-Desktop-PC-Tower-/361294273175?pt=LH_DefaultDomain_3&hash=item541ed11a97 , its a mini PC so unfortunetly they are horrible to customise. I want to play my steam games on it, like CS:GO and GTA V. I m wanting a graphics card that will fit the mini case (preferably a 3GB or 4GB card, im aware it will bottleneck because my cpu is only 2.4ghz), and im presuming ill also need a new psu, which will also have to fit. I ve already ugraded the ram, do you guys know any graphics cards or psus that will fit that are cheap as possible (im pretty broke)

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