[Conan]

@Conan Maybe I'll just buy with same Mhz of my previous RAM, just to be safe without getting any problem.

Follow-up question: My laptop can max upto 8GB of RAM now the question is, which is much better? x2 4GB RAM or x1 8GB RAM?

2x4GB. This is better if your device supports dual channel ram. Each slot will have it's own channel so there isn't any scheduling between 2 sticks of ram.

@Abdul Sami
RAMs rarely fail afaik. It also doesn't have a fail over mechanism. If 1 stick becomes worn out it will crash the whole system.