What's your PSU, mobo & CPU?
You have Windows 8.1 (ewwwwwwwwwwww!), so...I'd recommend ClassicShell, if you stick w/ W 8.1 here. Since Metro UI and even revamped 8.1 UI just sucks. Heck, I use it on W10, since I ain't fond of the tiles crap. Used it back in the W8 days, still use it for W10.
Classic Shell -
http://www.classicshell.net/
You can install Speccy Free Version and look at all your specs.
Speccy Free Version -
https://www.ccleaner.com/speccy
Your PSU, mobo, and CPU should be able to tell you how far you can go w/ this thing - which, I expect, really won't be very far at all; especially if you want to do modern stuff. You could get away w/ turning it into an older games PC's that would be good for older GOG titles and other not-demanding at all titles (i.e. certain Indies), though.
So, for cards...maybe you want to look at mostly cards ending in x50 if you're going NVidia? Namely - i.e. GTX 750, 950, 1050, 1650, etc etc. Don't know if you want to go into the 1050 or 1650 range w/ that rig. You'll need something lower-powered since you probably have a weak CPU and PSU. I don't think you'll want to play doctor here and do massive surgery on it though; might as well go build or buy a new one.
If you're trying to find a GTX 750 Ti (2014 budget card!!!) - yeah, good luck; it's old. You are not going to find that at retail. You're likely going to find NVidia GTX 10xx and whatever the newest generation of AMD Radeons at retail - even then, you'll probably bottleneck the crap out of the PC, if you mix a new graphics card w/ old hardware like an old CPU. You might have to go hunting from users on Reddit, CAG, maybe Amazon, and other places; and see if any of your friends have any of these old parts lying around and are trying to get rid of them.
EDIT:
If you're trying to say learn how to replace parts, upgrade, modify, tinker, and stuff of this sort - yeah, this also could be a test-bed for you to mess around with, if you are trying to do that.