bitflipper
The only way to know if you have enough RAM is to check usage on a large project. If you've got memory remaining in reserve, and you're not seeing excessive page faults, then you're set.

Yep... 
As long as you have enough RAM for your most dense projects, adding more will not increase performance.
16GB is plenty for many users.
More extreme Vi users may need 32GB or even 64GB.
 
For dual-channel motherboards:
If you want/need 16GB, use two 8GB sticks... which leaves the option to move to 32GB without throwing away existing RAM.  If you add RAM, make sure to install identical sticks (this eliminates potential RAM compatibility issues).
 
For quad-channel motherboards:
If you're running a quad-channel motherboard, you're probably more concerned about performance than cost.
Go with four 8GB sticks (32GB total)... which leaves the option of moving to 64GB without throwing away existing RAM.

I ran an (quite) old Intel Quadcore (2.5Ghz), with 4Gb of RAM for almost a decade, before upgrading, with success. I had to freeze tracks regularly, to keep the larger projects "happy". But it did not hold me back, in any creative way. But I think this is one of the things, where more is better. I have 16Gb RAM now, and the only improvement I have, is no need to freeze track repeatedly (beside "snappier" SONAR).
Best of luck.

16G allows plenty of space for softsynth samples for me.  I am quite sure I would be hitting a wall if I had less than 8G.  My piano alone needs 1.5G.  Windows and background tasks use almost 3G on my machine.

Maybe thats why my Platinum is jumping and splurting all over the place. 
 
I will buy two stick of ram by the end of the week and take it from there...

Make sure the RAM you buy is matched to what you already have.
 
Better still, buy a new stick(s)

For any 64-bit machine, I wouldn't go less than 16GB at this point in time. Providing it is a current generation machine, that shouldn't be more than $150 or so USD.

What hockeyjx said.
 
Also, adding a SSD for (at a minimum) the system drive, with samples on a separate drive, would go a LONG way towards improving performance on most modern machines!
 
FWIW...

I used 8GB ddr2 on my previous computer and worked fine for most audio mixing. Sometimes it became a bit excessive in dense projects but it was also due to my processor being outdated and old. Just couldn't hack it anymore.
When creating instrumentals, if i loaded too many instruments itd start dropping and what not. 
 
I now built a new computer. Currently have 6-core processor and 8gb ram ddr3, and it plays my old projects that would crash with ease. I use maybe an 8th of the power it used to take. I also have 2 more sticks RAM on the way il have 16gb ram. But have yet to really test my 8gb yet.

As others have said, it just depends on what you're going to be doing in Sonar. I use Sonar for composing video game music, and I work 99% with sample libraries and synths, so I use up a lot of RAM on heavily populated projects. I was getting by with 12GB for a while, but when I started writing more complex orchestral compositions, even 12GB wasn't enough. I had to upgrade to 16GB. If you're planning on using a lot of high quality/high resource sample libraries simultaneously, you'll be better off with a lot more than 6GB :)

Doktor Avalanche
16gb is what you should be looking for as a minimum. If you have anything below 8Gb it's almost certainly a bottleneck.

I ran x 64 win 7 and sonar 8.5 to plat x64 for years with 4gb and never had any issues. I have now upped it to 8gb as I bought kontakt 5. Funnily enough having the same conversation on the Sound on Sound forum and Martin Walker said he has 8gb and has never reached a limit in ram.