I personally do not like LCD monitors for gaming (ghosting)... I still use my 19IN Samsung CRT. Although for LAN, a LCD is preferable.
To be honest, Corsair is a great company, no doubt there. Although, I doubt the ~150$ more is worth the price. The memory might be better, and you might get an average 1-5 FPS (if you're lucky) more but is that really worth 150$. In my opinion, no, since you probably won't be able to make the difference between the two.
Depends, you can overclock, for instance, by increasing the voltage (there's quite a few ways to overclock) in the BIOS. You can also use a tool (you can probably find quite a few on the internet,
ClockGen for example) to overclock your processor. If you're really not looking to overclock your processor, than don't bother with the Gigabyte DS3 and aim for the E6700. Although if you do intend to overclock, I'd buy a lower end version (E6300 or E4400 with the DS3) and overclock it.