Just to be sure, double check the specs for memory
http://www8.hp.com/h20195/v2/GetPDF.aspx/c04400038.pdf
Have you tried inserting the dimm in a different slot? I'm assuming the slots that held the original 4GB DIMMs are working, does the new DIMM work in either of those? When you go into the bios does the 8 GB DIMM show up? Sometimes you have to really push down hard to get the dimm to seat properly; you may also want to make sure the copper contacts on the dimm are clean, they should be shiny. Be careful handling the DIMM, they are sensitive to static discharge. Follow precautions against static discharge. And lastly see if the dimm works in another computer. I think it's unlikely the motherboard has a bad dimm slot but you never know. Hope this helps