The HT bandwidth and RAM:CPU ratio link will ultimately determine how fast your RAM runs. You can raise your bus speed all you want, and your RAM will run as fast as it can. If it can't keep up with the bus speed (after ratio is factored in), it will give you an error. If your RAM is faster than your bus speed (after ratio is factored in), then it will just slow down to match the bus speed. DDR2 800MHz is more than enough for Athlon 64.