1_No,DDR2 doesn't have a noticeable difference with DDR3,also the speeds don't effect the performance either.The reason that it takes sometimes to open is because of your HDD
2_No,it depends on the motherboard specs,on some motherboards you can run a RAM faster than CPU speeds
3_Already answered in 1
4_I don't exactly get it.
All i7 CPUs require DDR3 memories if that's what you are asking