Getting a performance boost depends on what you actually do with the thing. Run Activity-Monitor (Applications>Utilities) to see how much of your RAM is used. Most Mac stuff runs well on 4GB, but if you run memory intense stuff (Parallels-Windows, Video Editing, editing large pictures with photoshop) you should see an improvement. But even if you don't need the RAM, you should not see a performance penalty (i.e. it may or may not run better, but won't run worse).
Unless your computer is already slowing down to a crawl with the current configuration (when doing memory intense work), adding 2x 2GB should be enough (giving you a total of 8GB).
So, given your configuration, I'd add 2x 2GB to the free slots and keep the memory that is already there in the other slot.
You have to weigh the balance between increased functionality/usability/security of the new OS with the performance of the old.
You have to ask yourself the question "is the performance increase/decrease worth the added functionality?"
Better yet, "Am I going to notice the difference?"
Your CPU and GPU are not going to change so running comparative benchmarks against them with varying OS's tells you little. If squeezing out every nanosecond of response is key to what you are doing, then this is entirely a moot thread; you should buy a new machine.
It seems that you are running into compatibility issues because your OS is now too old. Upgrading to one that is pushing 4 years old now may not be the wisest move - especially if what you are doing centers around the web where the tech changes almost on a daily basis.
Here's my suggestion (and I have personally done this for a friend and they couldn't be happier) based on the fact you have a pretty good machine in your possession.
First, upgrade your RAM. I'm suspecting you are seeing some performance slowdown if you are running out of memory. Open up Activity Monitor and leave it open as you go about your day. If memory utilization is close to or at the top of its limit, chances are you are sending the overflow memory requests to the hard disk (swap). That'll slow you down for sure.
Officially, your Mac can to go to 8GB of RAM. Unofficially, it can go to 16GB. Get as much as you can afford. More RAM means more space for your OS and applications to work in and less time being "swapped" to the hard disk.
Next, upgrade your drive to an SSD. I can't begin to tell you how much a difference this makes not to mention battery life and less heat. You are going to want to do a nice clean install, not an upgrade here.
Finally (and this one is optional), don't throw away that old hard drive. There are kits available that can turn your DVD drive bay into a second hard drive adapter. You can have two HDDs in your MBP. You can even opt for two SSD's and mirror them (RAID 1) for redundancy or stripe them (RAID 0) for increased performance.
One more thing....
If the "eye candy" and the animations are not important to you, those can be turned off, thus freeing up system resources to be used for other things.
Bottom line is this...for a relatively tiny amount of money (and some surfing on ebay or amazon) you can extend the life of your computer a few more years with much more recent OS.
Best Answer
1067 Mhz DDR3 is what you care about. You can do it on your own. Apple provides instructions on their website and you can find it on many places too.
For installing the RAM you just need a screwdriver small enough for the screws on the bottom of the computer. After that ground your self or use plastic gloves and take out the old memory put in the new and you are done.
Manufacturers don't matter as long as the memory is 1067 Mhz DDR3 for laptop. Its better to go for well known brands I guess such as kingston,crucial corsair etc.
I bought a Crucial Memory Kit 8GB from amazon for about £35 same type of memory with the one you need and it worked perfect.