Upgrading RAM

Hi,
My laptop config:
Win7 Ultimate 64-bit
i7-3612QM,
6GB RAM DDR3 SODIMM 1600MHz (4GB * 1, 2GB * 1)
750GB HDD
1GB Dedicated Graphics /NVIDIA
To upgrade RAM, I've 2 options:
1) Replace the existing 2GB module with 4GB (to make total RAM of 8GB)
2) Replace the existing 2GB module with 8GB (to make total RAM of 12GB)
Which of these 2 options is better?
And does it make any difference (or any impact) to have 4GB + 4GB (same size) as compared to 4GB + 8GB (different sized modules) in the 2 slots of the laptop?
what if I don't get the same brand of RAM as the existing one in my laptop?
Regards

Using the same-sized RAM sticks supports dual-channel operation. Not a big deal, but I'd go for it.

Mixing RAM may or may not work. On a desktop PC, the RAM settings (voltage, frequency, and timings) can be manually set in the BIOS settings, but those options may not be available for a laptop.

The least risk would be to buy two RAM modules in a dual channel kit, but that would be the most expensive.

(Actually, the most expensive would be to buy a pair of 8GB SODIMMs for 16 GB total. Your laptop would support that.)

Thank you all for your replies.
I hope its not a big job to replace the RAMs (which is not scheduled for immediate replacement, but definitely in near future).

And I hope this does not require the laptop to be reformatted when you upgrade the mem. Just replace the 2 modules and you are good to go. Win7 will automatically detect its new mem assignment. Correct?

Also, what shall I do with the page file after I upgrade the RAM (16GB RAM is sufficient for the life of this laptop and probably for me too ......... as long as I go ahead with this machine !!)

The opinion about page file is not too clear on this forum (atleast what I've read until now in so many threads) .......... some say "kill" it with so much of RAM to standby, some say "leave it to Win7 , it knows better what to do (for sure it manages mem better than WinXP ...." ... some say "assign definite size as min and max so that the page file won't expand or contract out of these limits" and some say "never mind, Win7 will still allocate it as and when required even without your setting" ....

The RAM upgrade on the Lifebook AH532 looks fairly easy. A large panel on the back must be removed. It's held in by 4 or 5 screws. When the cover is off, most of the motherboard is visible. The 2 SODIMM slots are right there.

It's nicer than some laptops, where the keyboard must be removed to reach the second RAM slot. (That's not as bad as it sounds, but makes damaging something a little likelier.)

You don't need to do anything to Windows to get it to use the new RAM. If you don't want to use hibernation, I suggest turning it off in Windows. Otherwise, an amount of space on your system drive will be reserved equal to the amount of RAM. (Hibernation saves an image of the RAM onto the hard disk, and reloads that when the PC is restarted. It restores the operating state. but it's slow.) This is mainly important if your OS drive is an SSD. (Recommended.)

if you buy a new higher speed 8gb (1600mhz) then it is worth it specialy because ram is cheep these days, also dual channel(a 2 module kit) will give you a small performance boost