i read online they said owc is ok...i'm using muskhin.

No problem from my end... I've bought a lot of OWC RAM modules, previously for my MP, and now on my MBP, they work like charm...

But be careful with their selection of cheaper NuRAM. Though OWC carries them, it's not their in-house brand, and some users have reported problems using them.

1. Kingston KTA
2. try garage sales section, i came across a few sellers

I have completed my upgrade few weeks ago for my 2011 13" Macbook Pro.

Previous setup;
4GB RAM stock
500GB 7200 Hitachi
Mac OS X Lion

Current setup;
8GB Kingston KTA RAM
Intel 520 180G SSD in MBP Sata III
500GB HDD in MBP Optibay Sata II
USB optical drive (optical drive removed to external USB case)
Mac OS X Mountain Lion (clean installation)

Done upgrade in 1 hour +

ML starts in 7 secs, all application launched within 3 secs, mostly around 1 sec. Photoshop CS6 launched in 2 secs, iTunes 1 sec, Safari, Chrome & Firefox all in 1 sec.

Overall I recog it as a big leap upgrade, even surfing web using Safari 6 is a lot faster than Safari 6 in Lion at 500G HDD.

I still keep my bootcamp in 500GB HDD, but it is really crawling as compared to ML in SSD.

BlackMagic Disk Benchmark: 405 Mb/s read, 200 Mb/s write.

I'm a happy man

You can try a clean installation if you have spare time, result is unknown, might or might not be faster. Yes, do a time machine backup will do. Thats what I have done when migrate from Lion to Mountain Lion.

The 520 SSD I bought is installed with latest firmware 400i. I'm not sure if there are new firmware available for 520 now?

I moved my 500GB HDD to optical bay and MAC OS X allows copy user files directly from the 500GB HDD to Intel 520 SSD. Whole OS installation takes less than 30 mins.

Looking at the Apple Store, you could just see why. The only easy to upgrade laptop still being sold is the 13" Non Retina model. Everything that went retina is now basically, for lack of a better word, "unupgradable".

Part of it is Apple's drive to make everything thinner, so most modular parts are now being soldered into the motherboard itself which includes among others the RAM, and they are also fitting in PCIe based SSD for the speed. You lose everything else though.

So this thread will continue to wind down, pretty much the only discussion on this thread after this would be on the maintenance, upgrade and repair for the older models.
Shake gently, listen for any loose component sound (there shouldn't be any). If there is, better check first. After that, turn on and see if everything is allright.