The RAM is, indeed DDR3. My current processor is an i5 2400, 3.1 GHz, quad core. The motherboard will support the following i7 processors - Ivy Bridge quad core and Sandy Bridge quad core.

No go on the I7-4790K drop-in replacement.
Your I5 CPU is socket LGA 1155. I assumed you had a Haswell I5 (LGA 1150).
Best CPU for LGA-1155 socket is prob I7-3770K (Ivy Bridge) and a quick look shows it as kind of pricey (~$400). If you could find 3770K on Ebay or somewhere for $150 I would say OK but it is no where near worth $400 over the I5-2400.
See here
http://cpuboss.com/cpus/Intel-Core-i7-3770K-vs-Intel-Core-i5-2400.
 
If you have built PC's before I would say swap the motherboard, otherwise save up for a new PC.

I am thankful I leave spec'ing computers to the experts  Trying to follow this discussion makes my head explode...I think it's time to play some guitar, but not without saying I am always VERY impressed by the level of expertise exhibited in this forum. In the immortal words of Ali G..."Respect."

I run a non K 3770 on my desktop and it's still a great CPU. I don't think the diff between the 37xx and 47xx was that huge and getting a "K" CPU is really only worth the extra dough if you're going to overclock which opens an entirely new can of worms. I haven't looked but I'm assuming the 3770 is more affordable, assuming it's still easily available.

My rule of thumb is that more RAM will only help if the system is currently short of RAM. If task manager shows a steadily growing page file, with lots of disk trashing or even pauses while Windows shifts stuff between page file and RAM then get more RAM. Task manager showing almost all the RAM is used but without swapping going on isn't necessarily a problem. A lot of that "used" RAM may well be there because it was being used for someting, but Windows hasn't cleared the memory locations yet. Which it will do in fractions of a millisecond if the RAM is needed for a currently active application, driver, whatever.

Equally it's unusual for Windows not to put some stuff into the paging file. Maybe stuff connected to drivers which are loaded but not in use, or things like Acrobat's "helper" resident tasks or a third party antivirus. The thing to watch for is swapping going on at a level that interferes with what you are doing. At that point, or just before it if you are sure it's going to happen when you load that new sample library, put in more RAM.

A faster cpu, on the other hand, will always return a performance increase. As, up to a point, will more cores and hyperthreading. But again, only if the current cpu can't keep up. Personally I'd build any DAW around an i7 for preference every time, but my DAW use involves audio, lots of dx plugins but few software synths or samplers other than AD2 and sometimes a wavetable synth. Big orchestral libraries and similar RAM hogs aren't part of what I do, but cpu hungry real-time monitoring of lot of effects are.

For what it's worth, I think my Sonar usage would mostly work as well in 8GB RAM as it does in 16GB. I can't say an i5 would do as good a job as the i7.

Page file may be an issue. I recently increased the size of my page file. I do have it static, not dynamic. When I run Latency Mon, the page file is the only thing that is even close to being high. Can you run with no page file? I admit, I'm not real savvy on that.
OOPS! I meant page fault in Latency Mon!!

You can run without a windows paging file but windows doesn't like it and slows down without one. If you have an SSD, set the paging drive on your SSD at twice your current RAM for the min and max size. This makes what was once known as a RAM drive and helps a lot. The size is subjective but twice was once the common setting. Ultimately, setting the page file on a SSD separate from the OS drive is optimal.
 
If you don't have an SSD then try to put the paging file on a drive separate from the OS drive (different drive channels). Use the same settings as above.
 
Even locking the page file size down to the OS drive helps a little. FYI, locking down the page file size stops some disk fragmenting due to the changing page file size. Also, windows won't resize and possibly rewrite the page file at whim.

DrLumen
You can run without a windows paging file but windows doesn't like it and slows down without one. If you have an SSD, set the paging drive on your SSD at twice your current RAM for the min and max size. This makes what was once known as a RAM drive and helps a lot. The size is subjective but twice was once the common setting. Ultimately, setting the page file on a SSD separate from the OS drive is optimal.
 
If you don't have an SSD then try to put the paging file on a drive separate from the OS drive (different drive channels). Use the same settings as above.
 
Even locking the page file size down to the OS drive helps a little. FYI, locking down the page file size stops some disk fragmenting due to the changing page file size. Also, windows won't resize and possibly rewrite the page file at whim.

Good point,
if you don´t already run a ssd drve for your OS and DAW
this is the component that will give you the most benefit .

If you have sufficient RAM and never run out of RAM, then the pagefile never gets written to. At the end of a session, if you look at the timestamp on the pagefile and it shows the same time as when you last booted up Windows, then that means it hasn't been used at all during the session.

microapp
Check the RAM usage in the performance monitor and/or Task Manager to see if you are anywhere near 8GB.
Unless you are running several instances of Kontakt or some other sample based synths, I doubt you will even notice doubling the RAM.
+1 to the I7-4790K. Still the best bang for the buck. I built a 4790 PC for my friend's studio this summer and it rocks.

^^^This. More RAM will only help if it's a bottleneck now. Without using samplers, it's unlikely you're tapping out the 8 GB you already have, in which case adding more will do nothing for you. Sorry.
 
A faster CPU, OTOH, will allow more tracks and more effects before it runs out of steam, and video may be smoother. 
 
However, the same principle applies as with memory: there's no advantage to having more than you need. It's not a video game or a sports car. If your i5 has no problem keeping up now, and you don't plan on starting significantly larger projects, then you may be disappointed by a CPU upgrade as well. Windows will boot faster, SONAR will come up faster, but you won't notice any difference in your recording projects.
 
It comes down to whether or not your current system is ever NOT up to the task, e.g. you're having to freeze tracks to avoid glitchy audio, you avoid certain plugins because they cause dropouts, or you want to watch YouTube videos while you're recording (not recommended). 
 
Here's a thought: if you're looking for a cheap upgrade, consider adding a 1 TB external disk drive for backups. One you can disconnect and stash elsewhere in case your computer gets stolen.