For me the hardware that made the biggest difference in usable low latency was dumping the Nvidia GT630 for an AMD Radeon R7 240.  Even after all the basic tuning (BIOS, no Windows 10 tunes) I would get an occasional click even running just a single instance of Ivory with EZDrummer and Trillian. Looking at LatencyMon I would constantly see the Nvidiakrnl (something or other Nvidia) as the culprit with the highest DPC routine and it would spike even though the overall DPC was very low and the programs graphs never went into, or anywhere near the "red. It was enough to give a highly intermittant soft click once in a while. I had none of the Nvidia garbage installed. Just the graphics driver and it was basically the same under Windows 8.x even with my Delta card.
 
Swapping the Nvidia with the AMD and bingo no more clicks at all. I've never been an ATI/AMD fan but in this case it was definitely a better choice for my set up with the x58a board.
 
Some days I wonder if I should just buy a Mac and be done with all this tinkering?
 
 

Yea, the Mac users are really struggling with El Capitan. I haven't followed it closely but the update really seems to have caused some serious problems for them with incompatibilities. Contrary to urban legend, Apple updates to major versions are not without their problems. The thing is, once the dust settles the Mac tends to work very well with a minimum of fuss.
I'm a tinkerer from way back but I tend to tinker when I'm pondering a new purchase and then after I work the bugs out, I leave it alone as I prefer to work on my music.
With regard to latency and oddities like DPC and clicks, pops etc it does seem that the Mac camp has a lot less problems than us Windows users. There are people running all kinds of resource intense plugins using MacMini or iMacs which on paper, spec wise, are blown away by even the most average Windows box. 
 
That's something that puzzles me to this day.

jbraner
It's probably because they control what runs on a Mac - where we have all kinds of (some good, some bad) drivers etc... ;-)

Absolutely!  The walled garden that Apple forces the end users into does have some advantages in that they pretty much control everything so chances are things will just work. Operating system upgrades not withstanding. 
So as long as the user doesn't mind doing things the "Apple way", it pretty much works.
Windows offers more choice and thus the potential for more conflicts and problems.
And Linux offers the most choice but unless it's for a server or a highly specific application, all of that choice leads to confusion and nothing working very well at all.
That's Linux's problem.