I used to run Win 7 x64 with Sonar x64 on 8GB which was totally fine for most applications, even 50+ tracks of heavy mixing with Steven Slate VCC etc.

When I started doing large-scale composing for TV I got frustrated with popping and latency when there are a bunch of synths open, and now I run 24GB. I don't think the indicator has ever showed more than 8GB in use, but I can keep 30 - 40 synths live with samples loaded and still get killer performance plus no latency.
 
My PC is only an i7 2600 (about 4 years old) with 1333 DDR3 RAM so I'm sure those numbers can improve tremendously by now, esp with faster bus speeds.
HOWEVER, the quality of your soundcard drivers may well make a huge difference too. I'm spoilt with RME :O)

pwalpwal
the op should see how it goes with 6, and add more if necessary

You can't really "add" in most scenarios. He will probably end up throwing away his current memory sticks and completely replacing them. Memory is cheap I would go for as much as he can afford. Over 16Gb is probably getting a tad excessive unless his gets into the sort of thing LJB is describing.
 
If he already has 6Gb he can monitor the situation via Process Explorer (link above), ignore all our advice and do what's best for him.

LJB
My PC is only an i7 2600 (about 4 years old) with 1333 DDR3 RAM so I'm sure those numbers can improve tremendously by now, esp with faster bus speeds.

I run similar spec's. I still can't believe how well it runs under my DAW environment.
 

LJB
HOWEVER, the quality of your soundcard drivers may well make a huge difference too.

 
Absolutely agree..

Doktor Avalanche
 
If he already has 6Gb he can monitor the situation via Process Explorer (link above), ignore all our advice and do what's best for him.

This is the best advice. Find out what is best for YOU.

Best Practice for Mac.....fill 'er up/max it out
Best Practice for PC......try to add at least 4gb to a stock PC if you have the slots.
 
Any rendering of audio or video needs anything it can get. 6(minimum)-8-12 will git-r-done. I had 2 4gb sticks so I just got 2 more..

Doktor Avalanche
If he already has 6Gb he can monitor the situation via Process Explorer (link above), ignore all our advice and do what's best for him.

What Alex said ^^^
 
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.

charlyg
Best Practice for Mac.....fill 'er up/max it out
Best Practice for PC......try to add at least 4gb to a stock PC if you have the slots.

 
Macs and PC's run on similar hardware so not understanding why the different advice.

I have 64 gb and regularly use about 40 gb of it; at that point the cpu seems to be the limiting factor in sonar, running all the vsts and isntruments, not the memory.

Macs use ram from top down. Microsoft has always been bottom up. Going to Intel and/or 64 bit may have changed things. I retired a few years ago....

charlyg
Macs use ram from top down. Microsoft has always been bottom up. Going to Intel and/or 64 bit may have changed things. I retired a few years ago....

I have no idea what this means but I'm guess you are stating that Mac's and PC's allocate RAM (physically and virtually) differently. That's a given but I'm not sure it's a huge difference (not a Mac expert but do know the hardware). I'm not sure though that buying more memory for Mac's makes their machines run any faster than Windows myself, just a different way of doing things... You can also configure Windows to allocate memory differently..