Yes, it will work. The board is an AM2 socket, and the Athlon II is AM3, and backwards compatible with the AM2 boards. (Not the other way around.) Your board is rated for a 89Watt processor and the Athlon II is a 95Watt, which is fine. Make sure that you use the Athlon II cooler.
So it may require a bios update, which I checked into and is not available from HP for that motherboard. A bios update specifically for AM3 socket CPUs' needs to be available for compatibility with an AM2 board.
I stand corrected, and apologize for my error.
Thanks whooleo. I should have checked this out before I posted.
Yes, but as for exactly which CPUs it depends. Looking at HPs site, there are different versions for that board, Narra through Narra6, and which one you have will determine what BIOS updates are available and what CPUs the board will be able to support. Seems like the original Nara though will support any of the Windsor cores up to the 5600+ that are 89W and below (no 89W 6000 Windsor for you) so you should be safe with that.