I am wondering if windows 7/8/10/superfetch is smart enough to load my entire os and/or applications folder (assuming it fits),
I doubt it does this. You could manually trigger something though. E.g. copy all the files to NIL/NUL. I assume that this will fill the disk cache with the relevant data.
But if you reallly want more IO speed, consider mirroring or striping SSDs.
and if it is is like to personally test the difference this makes (vs 16gb).
As long as you do not reboot windows (and any modern OS) will slowly start using free memory as disk buffers. So if you start a program the first time it might not be faster, but starting it a second time should see a speed gain.
(All assuming that you do not run out of free memory, which would cause disk buffers to be flushed).
If i do upgrade the ram, will it matter if I tack on different RAM to my current RAM.
No. It is fine to populate with different sizes, though if you mix then you might loose speed from dual channel access.
Examples:
One 2 GiB Standard start One 4 GiB Same with more memory Two 2 GiB Faster than 1x4GiB if your memory controller supports has dual channnel. (average 5% speed gain). Two 2 GiB ... of different speeds. Speed will slow down to the slowest comon values... Two 2 GiB and one 4 GiB .... This gets interesting and depends on chipset. In theorie the memory controllen can use dual channel mode on the first two and single on the second. Two 2 GiB and two 4 GiB ... dual channel mode on both. As far as I know always at the slowest shared speed. This is not a hard requirement and a smart chipset could use different speeds for both channels. (Adjust where needed for triple and quad channel. Those are not common in end-user land, but neither are Xeons.)
Not asked but otentially helpful: RAMdisks. Either for tmp folders or persistent ones which get their data filled after booting and whose contents can be copied on shutdown.
Also not asked but as a practical point: Measure. What is the slowest point? Simply adding more memory will not help [much] if it is not the bottleneck. And more memory can even slow you down due to less cache hits.