What motherboard do you have specifically?
The Phenom is AM2+, your motherboard would have to be AM3 if it has both DDR2 and DDR3 slots.
BUT - AM3 CPUs can fit an AM2+ socket (less pins), but not the other way around... generally.
The ability to use DDR2 OR DDR3 is limited to AM3 CPUs.
Phenoms ending with "0" tend to be DDR2. Ending within "5" are the next gen up, which could support DDR2 and DDR3.