The short answer is, if its not on HP's approved CPU list then it likely won't work. Just because the CPU can fit in the socket doesn't mean it will work. It sounds like AM3 support is not there. Also in question is whether the board can handle higher TDP processors. Otherwise I'd recommend the X4 940 for around $100, its AM2+ but also a 125W part.
Your upgrade options are limited. That's what buying an oem machine from gateway, Dell, Hp etc will do for you. Trying an untested BIOS on your motherboard is not wise unless you want to brick it and start over. Before sinking more money into this machine, I think you should weigh your options and possibly look at a system upgrade.