Are you 100% sure that's your motherboard? There are many versions of the M2N78-LA, and they all are slightly different.
1. The M2N78-LA (Viola) that you linked is a Socket AM2+ unit with the NVIDIA GeForce 8200 chipset. It only supports original 65 nm Phenom CPUs. The best CPUs you could put in this board are the 125-watt version of the Phenom X4 9950 Black Edition (2.6 GHz quad-core) or the Athlon X2 7850 Black Edition, which is a 2.8 GHz dual-core CPU. This unit has a brownish-yellow circuit board.
2. The M2N78-LA (Violet) is a Socket AM2+ motherboard with the GeForce 9100 chipset. It has a green circuit board and similar CPU support to the Viola board you linked to.
3. The M2N78-LA (Violet3) is a Socket AM3 motherboard with the GeForce 9100 chipset. It has a green circuit board and looks like the original Violet board, but it supports DDR3 memory and all Socket AM3 CPUs rated at 95 watts and less, with the exception of the Phenom II X6 series.
4. The M2N78-LA (Violet6) is a Socket AM3 motherboard with the GeForce 9100 chipset. It is identical to the Violet3 board except the bottom PCIe x1 slot is swapped out for a conventional PCI slot. CPU support is the same as the Violet3.
If you do have the yellow-brown M2N78-LA Viola board, you are limited to original Phenom-class CPUs, the best of which I listed above.