If you have an MSI K9A2 Platinum you can lock the NB and HT (*edit - Maybe not 'lock' the NB/HT but adjust them independently of the memory speed). You can also run a number of memory dividers to keep your memory within spec.
Is that the motherboard you have..? The MSI K9A2 Platinum? If you have some other version and can't lock those things then don't expect much overclocking headroom without a 'Black Edition' processor.
Again, assuming you do have the K9A2 Platinum be sure to have the latest BIOS if you can't find the NB/HT/Memory settings, it looks like
the latest BIOS is v1.6.
The settings you are looking for are in the 'Cell' menu in the BIOS.
*edit 2 -
K9A2 motherboards. - All of them have some kind of suffix. You just said 'k9a2' so I'm not sure which you have...