As target50 stated, an upgrade to an AM3 board w/DDR3 would be wise. Any 870, 880 or 890 chipset would do good. The 555 CPU may stabily unlock with a newer board. Memory prices are low. You can always upgrade the CPU at a later time.
I have an 890 board that I ran a 555 on. It unlocked and oc'ed to 3.6. It also ran well as a dual core at 3.6. Much faster than my 550 on a 785 DDR2 board.