[Solved] RAM compatibility with motherboard ??

If your board says 1.5V default (most DDR3 boards do) you CAN use 1.65V memory and it won't damage anything. But you will likely have to go into the BIOS and adjust the memory voltage yourself unless it autodetects and supplies the power correctly. Some boards are good at that, some aren't.

The effect of having not enough voltage supplied to your RAM is usually that you will get random freezes and/or crashes when you try to do anything memory-intensive, so getting that right is a must for smooth operation. But it won't ruin any components by being underpowered.

If you don't have a lot of experience with this kind of thing, sticking to 1.5V memory for now will probably be easier on you if it doesn't cost you anything to exchange. It's much more likely to just plug-and-play correctly on the first try.

To support more RAM, go with Vista 64-bit or Windows 7 64-bit. Those should both allow up to the full 16GB. But you may want to start out with just 4GB or 6GB anyway -- if you fill all four slots, you often run into additional quirkiness in order to get your system stable. So I'd save that for later.

If you put 1600mhz RAM into a board than only supports 1333mhz, all that should happen is the memory will just operate at 1333mhz.

I'm not familiar with that particular motherboard, but usually numbers like that (by which I mean the 2100+) means that it will support 800, 1066 or 1333 mhz RAM by default, and it can support anything between 1333-2100 mhz if you overclock it. Although with RAM designed for 1600 mhz, I'm not sure whether that really qualifies as "overclocking" so much as just entering the correct speed and timings manually.