So I saw this pair of ram on sale today for pretty cheap, but I already have a pair of rams. The one I'm looking at is a kingston pair 1600mhz with 4gb on each stick, and the one I have is a kingston pair 1600mhz with 2gb on each stick. Both stick have same voltage and everything. Would it cause problems if I have a pair of 4gb rams and a pair of 2gb rams?
It is important to have both DIMMs in a pair the same size, but having one pair of 2GB and the other pair of 4GB is fine - there will be no problem there.
Just make sure that you have the DIMMs in the correct sockets. On many motherboards the pairs of DIMM sockets are different colours. Make sure you have one pair in one colour and the other pair in the other colour (they will often be interleaved - don't let that confuse you).
Your motherboard manual should tell you where to put them.