OK we see from http://h20566.www2.hp.com/portal/site/hpsc/template.PAGE/public/kb/docDisplay?javax.portlet.begCacheTok=com.vignette.cachetoken&javax.portlet.endCacheTok=com.vignette.cachetoken&javax.portlet.prp_ba847bafb2a2d782fcbb0710b053ce01=wsrp-navigationalState%3DdocId%253Demr_na-c03254185-13%257CdocLocale%253D%257CcalledBy%253D&javax.portlet.tpst=ba847bafb2a2d782fcbb0710b053ce01&sp4ts.oid=5187028&ac.admitted=1405380528605.876444892.199480143 it has a 300 WATT PSU.
That's barely enough so you'll buy a 600 WATT PSU first then use the remaining funds to pick from this list.
The small case likely means you'll leave the cover off or cut about a 30CM square hole in the case cover to max out the air flow.
I have a HP Pavilion p6-2120ea Desktop PC and was wondering the best graphics card I can put in it thats cheap? Budget is £200