Building first pc, and I have a lot of questions about RAM choices.

I just bought a 955 BE + 890gx motherboard (this one) for a cheap, but future proof-ish, gaming computer.

But now I need to pick out RAM for it, and I had some questions I was hoping you guys could help clear up. I want 4GB at ~$100-110.

#1) DDR3 1600 or DDR3 1300? My mobo lists DDR3 1866 (OC)/1333/ 1066. Does this matter? Would I have to force 1600 to be slower, or could I make the mobo faster? Should I get 1333 over buying a 1600 and making it run at 1333? Would OC'ing add significantly to heat/noise?

#2) What is the difference between PC3 10666 and PC3 10600. I thought this was just rounding error, but G.Skill lists two products that look nearly identical with different model numbers.

#3) How does voltage come into play regarding choosing one RAM over an other? My mobo specs as using 1.5V DIMM sockets.

#4) Is it a big deal to add 2x1GB sticks later to bring me up to 6GB with a dual-channel kit? That seems like a cheap upgrade for better performance (I use virtual machines often). Would adding a single 2GB stick make for poor load balancing/poor performance?

#5) Lastly, any kits you guys recommend? Other companies I should look at?

Thanks.