I have read a number of posts on this website and they always seemed to have good information in the past, but both of the previous posters really should have researched the motherboard configuration before responding.
The Asrock n68c-s motherboard supports
both DDR2 and DDR3 ram. It has two slots for each, and can hold up to 8 gigs of DDR2 or DDR3 (Read also as much as two 4 gig sticks of either one). You cannot run both DDR2 and DDR3 ram at once on this board, you must choose which type you want to use. That being said, I assume the OP's question was regarding how to switch over from DDR2 to DDR3 on this board. All you should have to do is power down the system (make sure it is also unplugged and that you are working grounded!) and remove the old DDR2 memory from the motherboard. Locate the slots for the DDR3 ram, they are color coded. I think the blue is DDR3 but I may be wrong, so double check before attempting to install. Be sure to confirm that the ram is in the right slot for the type you choose or it could damage your motherboard.
Another important thing is making sure the particular stick(s) of ram you choose are supported by that board. I have the same motherboard and if you pull it up on the Asrock site, it will give a sidebar option to see a list of supported memory. This will tell you what size, make, and model your board will run. I recently bought a stick of four gig ram and was surprised to see that only two different four gig DDR3 sticks are even supported, so be sure to check this out. As always, if your system fails to boot after the switch, double check that the memory model number is compatible. If it is, the sticks are probably not seated properly, or there may be an option to change in BIOS. Power down, remove, and reinstall to confirm that the ram is seated properly. There is also a tech support number for Asrock if you wind up needing help getting it to run. It is not a 1-800 number, so be sure to request that the tech support rep call you back, usually companies will happily do so and this way you do not continue to get charged for the call.
So in short, you cannot run DDR3 in a DDR2 slot, which you probably already knew. However, you can run
either DDR2 or DDR3 on this board, just not both at once. So for switching from running the older DDR2 to the running newer DDR3 just follow the above instructions. Make sure you use the proper slot and that the ram is listed as compatible and you are good to go!
Just a side-note, I have no idea how old this thread is, but since there appears to be some confusion about this board's capabilities, I figured I would add some clarification. Who knows, someone else may stumble upon this forum needing the same question answered.