I'm currently using an Asus P5LP-LE motherboard with a Pentium D 930 (3GHZ) and haven't had any luck on the HP forum with regards to finding out specifically what newer processor/s would run on my current motherboard.
According to the HP product info regarding my motherboard it doesn't show/list my current processor specifically. It shows the following info:
Your motherboard has the 945p chipset; it won't work with any core2 duos. I suggest against an upgrade. You won't notice enough difference for the $$ spent. If you want to use a core2 duo cpu, checkout ebay for the "boston" hp motherboard, which I used to own. It will work with most core2 duos, using ddr2 pc 5300 or pc6400.
Apparantly, your hp system may use more than one motherboard. Use cpuid or everest to id your motherboard chipset. If you have the older 945p and want to change boards, ebay is your only affordable source, unless you get lucky and find one on craigslist.