your PC have PCI-E 16x and all PCI-E motherboards support all PCI-E graphics cards
but you need new PSU to support high end graphics cards, the best graphics card you can buy for your PC for 300w PSU is AMD HD7770
here are your PC specification http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=120&prodSeriesId=3658372&prodTypeId=12454&objectID=c01759467