my friend has this pre-built: http://support.hp.com/us-en/product/HP-Pavilion-500-Desktop-PC-series/5399647/model/6871733/document/c04121441/
so are any of these GPUs compatible with that if he upgrades his PSU:
http://eu.msi.com/product/graphics-card/R9-380-GAMING-4G.html#hero-overview
http://xfxforce.com/en-us/products/amd-radeon-r9-300-series/amd-radeon-r9-furyx-liquid-cooled-r9-fury-4qfa
http://eu.msi.com/product/graphics-card/R9-380-GAMING-2G.html#hero-overview
so are any of these GPUs compatible with that if he upgrades his PSU:
http://eu.msi.com/product/graphics-card/R9-380-GAMING-4G.html#hero-overview
http://xfxforce.com/en-us/products/amd-radeon-r9-300-series/amd-radeon-r9-furyx-liquid-cooled-r9-fury-4qfa
http://eu.msi.com/product/graphics-card/R9-380-GAMING-2G.html#hero-overview