bruce20 :
Legohouse :
Speed, go with 1600Mhz if your laptop supports that and if you are opting for 2x4GB. Else if you are going with 1x4GB then go with 1333MHz that's the first priority. However, you can try a single 1600MHz 4GB module along with the existing 2GB, in this case the 1600MHz will run at the slower speed (1333MHz) instead of its theoretical bandwidth and still it will run in Dual channel mode but it can be a little complex with your laptop.
As far as mixing size is concerned, the memory will work in what is called a "Flex Mode" - 2GB out of the 4GB and the present 2GB module which will run in Dual channel mode, while the remainder of the 4GB module (which is the other half of 4GB) will run in single channel mode.
In anyway, the performance gained by using dual channel is only 10-15% over single channel but this might come in handy if you are trying to push the performance to its peak.
My suggestion is go with 2x4GB, the price of the modules are not that high and you will notice the difference in performance. Btw, make sure you have 64Bit Operating system (windows), 32Bit OS doesn't recognize above 4GB (well not above 3.5GB actually).
P-S: If you can lemme know your HP product number. Check under the laptop or inside the battery compartmenr.
OK Cool, really appreciate your detailed answer. Thanks.
I'll check my exact laptop model soon & post here. By the way, I read somewhere that 1666MHz ram is CL10 or CL11, which is slower than 1333MHz ram, which is actually CL9. They mentioned CL is the processing time of the ram, the lower, the better. What you think about this?
Thanks.
To put it on simple maths:
The frequency is expressed in Hertz, which means "cycles per second".
So, the DDR3 1600 will perform 1600 cycles a second while the DDR3 1333 will do, well, 1333.
The CAS latency is given in cycles. So, a CAS9 RAM will take 9 cycles to respond and the CAS10, 10 cycles.
Now putting it together: the DDR3 1600 CAS10 will take 10/2000 seconds, which is equal to 0.00625 seconds, to respond while the DDR3 1333 CAS 9 will take 9/1333, which is equal to 0.00675 seconds, to respond. Thus, the 1600 one is faster.
FYI, DDR3 1600 has CAS 9 and CAS 8 memory too but CAS 9 is much more reliable.