Ivy bridge cpu's are not sensitive to ram speeds, 1333 is fine. Read this:
http://www.anandtech.com/show/4503/sandy-bridge-memory-scaling-choosing-the-best-ddr3
Most laptops have two ram slots. You could match your current ram with another stick in the second slot to give you 4gb.
Download and run cpu-Z to see exactly what ram specs you are using. Adding a second stick will give you dual channel operation which speeds up your cpu processing a bit. Likely, you could add a 4gb stick to get a total of 6gb.
The odd 2gb would run in single channel mode.
A 32 bit os can only access a max of 4gb. Only go higher if you have a 64 bit os.
Ram is sold in kits for a reason.
Ram from the same vendor and part number can be made up of differing manufacturing components over time.
Some motherboards can be very sensitive to this.
That is why ram vendors will not support ram that is not bought in one kit.
Although, I think the problem has lessened with the newer Intel chipsets. Still,
it is safer to get what you need in one kit.
You want documented ram compatibility. If you should ever have a problem, you want supported ram.
Otherwise, you risk a finger pointing battle between the ram and motherboard support sites, claiming "not my problem".
One place to check is your motherboards web site.
Go to a ram vendor's web site and access their ram selection configurator.
Enter your laptop make and model, and you will get a list of compatible ram kits.
While today's motherboards are more tolerant of different ram, it makes sense to buy ram that is known to work and is supported.