DDR3 doesn't go as low as 600MHz. It's likely that you're getting slightly confused between the actual clock frequency and the DDR frequency - the latter is twice the former, as it's Double Data Rate memory. So for example, 1600MHz memory will be clocked at 800MHz, some programs read this frequency while others will read 1600MHz.
You will need to set your memory to its rated speed, timings and voltage in the BIOS, yes, I always suggest doing this rather than leaving it to be set automatically. Most memory will include a profile to do this, the procedure to load this will be detailed in your motherboard manual if required.