off the subject; there should be a tax on people who buy slim line PCs and then want to game on them.
on the subject; try the Nvidia GT 640 low profile card. it hardly draws power and will run diablo III fine at medium settings at resolution like 1600x900. while AMD has some better performance cards at low efficiency, they are all PCI-E 2.1 cards and can cause issue with older mobos with PCI-E 1.0 slots that rely on mass vendor BIOS like HP.
off the subject: don't shop at best buy.
GT 640 low profile cardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814121640