Computer freezes after upgrading of memory

Hi,

I have recently upgrade my ram from 2GB to 6GB.
Motherboard that I using currently is ASUS P7H55-M LX
It basically has only 2 slot. Intially I bought a 2GB ram and wanted to increase it recently. Did not thought of wasting too much on ram since I have bought it only a year ago.
Hence I bought a 4GB ram and placed it in.

Here comes the issue:
After placing it, my computer keep freezing.

So my question is that is it alright to mix ram size in the same motherboard?

PS: Both memory are Kingston and they are Kvr1333d3n9/4g and kvr1333d3n9/2g. I have checked that they both have the same speed, same voltage.
Hence I would like to know if my ram is the main cause of my computer freezing or other reasons.

Thanks for the help..

Hmmm, this is not my forte but, have you tried to use only the 4GB stick on the Primary slot (I think its the closest to the cpu) to test if it works on its own? if it does, then shut down the rig and insert the 2GB on the secondary slot and test it again... that may do the trick.

If it freezes again, then it is easy to see that there is a conflict with both sticks and/or your Mobo has the capability for more than 4GBs?