Might as well go ahead and make the AMD plunge.
Two MATX mobos on NewEgg with a five egg rating... the BIOSTAR TForce6100-939 Socket 939 NVIDIA GeForce 6100 Micro ATX, and the SUS A8N-VM CSM Socket 939 NVIDIA GeForce 6150 Micro ATX
Only about a $13 diff in price, so either would work.
To use dual channel, try to amtch your RAM exactly for best results.
As for a CPU, buy what you can afford.
Look for a new PSU as well. Your HP probably has either a 200W or a 250W. Neither one will cut it for 64 bit and a huge graphics card.