My brother who's a tech-guy built me a sweet computer with a lot of good parts, but after progressing more and more with my own music, it's became where I'm having 16+ vocal tracks on Sonar for a normal song- and with all the plug ins running and such it lags like crazy. I've been trying to mix a new song and for the first 30 seconds of it being open, it can handle it- then it just lags away. The audio skips, the line going across the project gets stuck, it sounds distorted from the lag, etc.  I can't mix it now nor play it back to hear how the whole thing sounds. 
 
Any suggestions? Even my brother was unsure as to what sort of upgrade I'd need to be able to run such big projects. RAM? Better processor? Sound card? I need some advice
 
Thank you
 
Specs I know off hand:
 
Processor: AMD Athlon(tm) X4 860K Quad Core Processor 3.70 GHz
Installed RAM: 8.00 GB
64 Bit Operating System
Scarlett 2i2 USB

First thing, when you are mixing (done recording) are you increasing your buffers to the highest setting?
 
If you are still recording, I would bypass all effects (or as many as possible.)

Which buffers are you talking about? (vocab noob here)

Your ASIO buffers on your interface.

I've turned the ASIO buffers to the proper settings, someone on a similar thread recommended that, but it did not help whatsoever

16 tracks should not be an issue with those PC specs, but the number and type of plugins could be a deal breaker, depending on what all you are running.
 
If you are using any plugins with linear phase, such as Cakewalk's LP-MB or LP-EQ, they must be used in non-linear mode while tracking or mixing.  Linear is best used when doing your final mastering.
 
Some plugins could just be very CPU hungry, in that case you could bounce down a few tracks to lighten the load.
 
Or try disabling your plugins, and then enable them one at a time while observing your PC performance.  Maybe you can identify some that are causing your issues, and if any of them are particularly troublesome in your rig, then you might try a few alternatives.

You could try "freezing" tracks. That applies all the effects and processing to a track then inserts a copy of the resulting audio into the track and switches off all the fx on the track. Which means those plugins are not longer using cpu resources. To revert the track to the state it was before frozen so you can work on it again just unfreeze it.

It slows down working a bit because of the time needed to freeze or unfreeze tracks, but it was pretty much essential in the days before modern multi-core fast cpus.

Other than that, a faster cpu is probably the way to go. The faster the better, and personally I'd go for Intel rather than AMD though that might just be my prejudice, the last time I used AMD cpus was many years ago when they were better performers than Intel. Probably back when cpus had just the one core.....