From what I can find your CPU is Socket 939 (I was thinking it was AM2). This makes it worthless to upgrade the CPU, since 939 is 3 Socket types behind now.
Options:
Buy a new motherboard and CPU - AMD (~$200)
Buy a new motherboard and CPU - Intel (~$260)
Buy a new CPU and be just as bottle-necked. ($25-100)
Wait and buy all new Board/CPU, RAM, HDD, DVD and other parts to replace (Price varying)
Your RAM is currently DDR2, so even though it's slower speeds you won't need to replace it if you go the Motherboard option. The only problem I see maybe happening is HP being annoying like normal and having made the Motherboard some funky shape so as to be un-upgradeable. If that is true, which I hope not, you'd need a new case, which can run anywhere from $25-200+.