!! First of all, THANK YOU to so many for being so helpful !!
Well, since my largest project is only showing 66% CPU usage and 39% Memory usage, it would appear that my system is still sufficient for my needs at this time. That said, I will upgrade to a new i7 system when I can, just because. Having recently retired, my income isn't what it used to be so, it will be a while but, it will happen eventually.
In the meantime, here's something that drives me crazy and, I don't know if this is related to memory, or processor, or what. When I stop a project that is playing (space bar or hit stop), it takes it a second to actually stop. This makes Sonar seem sluggish or laggy to me.
Also, when the project is playing and I change positions in the song (click in time line or click to a marker), it often creates a click or pop when it restarts at the new position.
I don't know what causes this to happen. This is what originally led me to start questioning my system in the first place.  

The delay after pressing stop is most likely a large buffer setting and/or a high latency plugin. Linear phase plugins are a likely suspect. Hit the 'E' key to temporarily disable plugin effects and see if the delay goes away. If not, lower your buffer setting a few notches and try again.
 

Disabling all FX didn't change anything. I will mess around with buffer settings.

rodreb
!! First of all, THANK YOU to so many for being so helpful !!
Well, since my largest project is only showing 66% CPU usage and 39% Memory usage, it would appear that my system is still sufficient for my needs at this time. That said, I will upgrade to a new i7 system when I can, just because. Having recently retired, my income isn't what it used to be so, it will be a while but, it will happen eventually.
In the meantime, here's something that drives me crazy and, I don't know if this is related to memory, or processor, or what. When I stop a project that is playing (space bar or hit stop), it takes it a second to actually stop. This makes Sonar seem sluggish or laggy to me.
Also, when the project is playing and I change positions in the song (click in time line or click to a marker), it often creates a click or pop when it restarts at the new position.
I don't know what causes this to happen. This is what originally led me to start questioning my system in the first place.  

I know it sounds silly but are you using a wireless keyboard ? replace the battery.
there also used to be a function where if you pressed the Pause/break button it would slow down resources.
(graphics).
probably not your problem but its always best to over turn every stone before moving on to the bigger stuff

Adding additional unused RAM will have *zero* performance benefit.
Upgrading the CPU could offer some benefit (especially if you're pushing the limits of your hardware).
 
I wouldn't spend a lot to upgrade using the current RAM/motherboard.
That money would be better spent toward newer hardware.

+1 for the SSD.
 
 

Gosh its complex this PC stuff aint it!
 
I've had a quick scan of the threads so far, but soz if i'm repeating stuff.
Here's my twopeneth..
 
I'm just gonna mention Benchmarking CPUs

• I've had an i5 4440 on a LGA 1150 socket since 2014 (which benchmarks on cpubenchmark.net at 6476) it's performance is fine, but like rodreb's it struggles with multiple tracks hungry plugins
• I also have a laptop with an i5 3337 (which benchmarks on cpubenchmark.net at 3213) it can only handle basic sonar stuff
• I installed an SDD to my music pc which made quite a difference in load times
• I upgraded my ram to 16gb - difference was negligable

I'm thinking I might upgrade to an i7 (Intel 4th Gens) (need be sure what your Motherboard supports) a new 7th /8th gen i7 wont physically fit, it will have to be a forth gen like my current one.  Can be picked up second hand around £120 -200.
 
As an example option - an i7 4790K benchmarks at 11191 (Almost twice the processing power)
 
With so many CPU variants, I ALWAYS check cpu benchmarks before I think of buying.
 
It's true there is much to what makes a pc robust enough for audio processing. Things like bios settings, ram speeds, Ram drives, SSD drives n stuff, but the biggest 'bang for buck' as they say, for me, comes from cpu performance.
 
I couldn't afford an i7 when I built my pc, but now... well it's doable
 
if you want to see how your cpu benchmarks just google the cpu name like i've shown together with the word 'benchmark' and you will no doubt get a link direct to its performance page
 
FYI - It might be obvious but for the sake of it ..... when you're looking about... Notice that 4th gen cpu' have a number staring with 4---, 5th gen's have a 5--- number - etc... always check the socket type and mobo compatibility  
 
Hope this has been of some help to somebody

But more horsepower is always better!

I have to echo that if you don't have an SSD, upgrading to one is likely to be your biggest bang for the buck by far.
 
With 8 GB of RAM and what you said so far, more RAM would likely be throwing money away with no benefit. 
 
Keep in mind that "66% CPU usage" does not necessarily mean that CPU speed is not a problem as this is based on overall usage for all logical processors. Not everything can be easily split across multiple logical processors so you can be peaking out on a couple of of them while the rest are sitting mostly idle and useless. If you right click on the CPU graph in Task Manager, you can change it to show graphs for each logical processor and that will give you a better idea of whether you are running into a raw CPU speed limitation. 

Wow, some terrible advice in this thread...
 
Anyways, adding RAM will do NOTHING if you're not even using all that you have now. Running at 66% of your CPU power does mean that getting a significantly faster chip would most likely improve the response and smoothness of your system. I've run HUGE 80-track projects with 4GB of RAM BTW with no problem; it's mostly big samplers and some softsynths that use a lot, not really other plugs. As a matter of fact, I don't think I have a project that causes Sonar to use more than 2GB for itself. My little laptop has 4GB and loads any project I've ever made without RAM problems; now its dual core U series i3 can't handle the plugins but that's another story.
 
Now, for what you can ACTUALLY put in your motherboard (LOL on some of the responses here...): used i7-3770's go for $120 on Ebay; depending on what heatsink/fan you have now you might have to get a new one of those. It's not THAT much faster than what you have, but it should make a noticeable difference in smoothness of operation. The K version is not worth it, unless you have a Z-chipset motherboard with which you can overclock it. The built-in GPU is a lot better than the one in the i5-2500 too by the way!
 
R