Does this mean that I must overclock, or simply CAN overclock?
http://www.asus.com/Product.aspx?P_ID=6nnVb6RBxd7PhGmt
Asus writes the spec for this motherboard as:
Dual-Channel DDR2 1066(O.C*)/800/667
Does this mean that I have to overclock to support 1066Mhz RAM, or does it mean that the RAM is overclockable only if it is stock 1066Mhz?
Yes, it means that if you want 1066MHz DDR2, you’ll have to overclock the FSB to 1600MHz—1333MHz is the maximum supported speed—(if the RAM is already 1066MHz stock, then it won’t be overclocked; that’s the maximum speed that board can do for RAM). You can see the details in the Specifications tab of the page you linked to.
You will need to overclock the FSB to 1600MHz in order to use the 1066 DDR2 RAM at 1066MHz. If you use the rated maximum of 1333MHz, the RAM will run at 800MHz, not 1066MHz.
So your options are to either OC the FSB and run the RAM at full speed (assuming you don’t already have slower RAM installed in which case all the RAM will run at the speed of the slowest stick), or else run the 1066MHz RAM at slower than its maximum (800MHz).
To be fair, if the mobo mfg tells you that you can OC the FSB, then it is likely that they have tested it at that speed and found it to be relatively stable enough to so much as mention it. If it was really unstable, they would never include it in the site or manual. Even so, if it is a new board and you are concerned about the warranty, you could drop them a quick email to ask if it would be voided if you overclock (make sure to mention “like is says on your site” and “like it says in the manual” to put the onus on them).
Another option would be to just buy 800MHz RAM instead of 1066MHz and run the FSB at its rated maximum of 1333MHz. However that would only be a better solution if the 800MHz RAM is cheaper than the 1066MHz. If you can find 1066MHz RAM for the same or lower price, get that! Not only will it run at the lower speed without issue, but it will allow you to run at the higher speed should you decide to try overclocking or get a motherboard that supports a higher FSB without overclocking.