These are my thoughts for the upgrade: CPU -   i7-4790 w/stock fan. GPU -   GTX 1660 super, RTX 2060, RTX 3050 PSU -   can be upgrade with a standard ATX, 140 mm depth for exact fit.  Although the OEM 460w should be sufficient enough for above upgrade. SSD -   Beside mSATA, 2.5" SATA solid state drives can be used w/adapter. Fans -  A 92 mm fan can be added to bottom front existing fan mounts.  Use molex or sata power.
TLDR:XPS 8700 I5 4440 / Dell 0KWVT8 MOBO Intel LGA 1150 CPU Socket / GT 635 GPU / No upgrade to HDD (whatever comes standard with model) / 460w PSU / 16GB of ram. What can I do to make this pc a better gaming machine without changing the MOBO? I was given this desktop for free from my father who had it stashed in his closet for a vey long time. I am using it as a low end gaming pc and don't want to buy/build a new gaming pc anytime soon, but I would instead like to use this as a learning tool for upgrading hardware on computers so when I eventually do build my own gaming pc I have some experience. CPU- Best Compatible CPU available? Will I need to upgrade CPU fan? If so how do I do that without causing errors on startup?GPU - I believe this pc can handle up to a 1060 without upgrading the PSU, however If I am upgrading the CPU&PSU anyways is there anything higher than a 1060 I could upgrade to based on the available CPU options? Will I need to upgrade the exhaust fan at all when I upgrade the GPU to reduce temps?PSU - I am sure I can swap this to anything that is compatible with power connector on the Mobo? SSD - Do I only have the option of an Msata with this model or can I use a more modern SSD? Will I need to have some sort of adapter for housing the SSD in this model? Fans - If cooling becomes a concern/problem after installing these upgrades are there ways I can use aftermarket fans to help with cooling without causing any errors?  
XPS 8700, how to go about upgrading this dinosaur