Do you need the current windows operating system? If so, then shop for a newer hp chipset board on ebay; if you already have a regular windows dvd, then go for a 785 or 790 chipset board. Your main choice is whether to use ddr2 or ddr3; I would spend the extra $20 at this point and get a 2x2gb kit of ddr3 at the best price you can find for corsair, crucial, kingston, pqi, pny, gskill, or samsung. Those are the brands I trust.
As far as I can tell, you will have to look elsewhere to find mobos that support DDR memory and/or socket A processors. If you don't mind upgrading both (which you probably would, since then you would have a 2/3 new computer) the task becomes much easier. So your options (in whatever order): 1. Abandon project 2. Upgrade CPU, mobo, and RAM 3. Look at a different e-tailer.