What memory speed should i buy?

[garukno]

lately my computer is so slow and i want to upgrade my memory but im confuse which one should i buy
a single channel 8GB 1866MHz or dual channel 8GB 1600MHz?
i really want the single channel so i can upgrade to 16GB in the future since my motherboard can only support 2 stick
but my friend keep telling me to buy the dual channel memory

edit:
my pc component
CPU: AMD A8-7600
MB: ASRock FM2A68M-HD+
RAM: Team Elite Plus 4GB 1600MHz Dual Channel
PSU: Corsair VS 450

 

[Hellfire13]

Dual channel runs slightly faster than single channel. Hence its always recommended. That said, single channel will work as well. The flip side is, if it fails, you wont have another stick to boot with.
Also that psu is not very good.