XPS 8700, upgrade options

I have an XPS 8700 that is 4 years old.  I have a laptop that's about a year old and I hadn't been using the XPS 8700 much as it is in the basement.  Then work from home hit and it got me thinking about whether there are any cost-effective upgrades I can do to my XPS.  I'm not terribly technically proficient but can follow directions quite well.

My XPS specs are:

CPU - Core i7-4790

RAM 16gb

HD - 1 TB HD

GPU - nvidia GTX 745

I use the computer for work, some gaming, and to host a Plex server with my movie collection on an external HD.  I was thinking about adding an SSD and perhaps upgrading the video card.

Questions:

1.  What SSDs can this computer use and how much of an undertaking is it to add one.  If I want to use the SSD as a boot drive, what's involved in getting Windows onto it?  I assume that doing this would be a reasonably significant improvment in boot time and general operating performance.

2.  Are there any video cards that would be a reasonable upgrade that wouldn't require anything other than swapping out the card?  I can do that, but I suspect that something like upgrading the power supply would be beyond me.  I don't do a ton of intense gaming or anything like that, so I'm kind of on the fence on this one to begin with.

3.  Any other upgrades that I should consider?

Thanks!