Thanks for all your help, I'm revisiting this idea now that its been on a back burner for a few weeks.
My biggest hesitation is in knowing that the actual computational render time is most effected by the CPU, which I'm not upgrading with this plan. The SSD and RAM will help improve the overall computing experience as I understand it, but likely will have very little improvement over actual rendering times. The USB3 card will help with only very specific operations (backups mostly). The video card will boost certain operations during rendering (especially when scaling with Premiere), but at the end of the day a CPU upgrade I think would make the biggest difference in a return-on-investment for work purposes.
So while I like the idea of upgrading components and keeping this box alive for a bit longer (for mostly economical reasons), there's a voice in the back of my mind telling me to save it up for a setup with a faster CPU.
So that's got me thinking- Its been literally over a decade, perhaps even closer to two since I built my last custom PC. Back then, it was relatively easy and economical to swap CPUs and motherboards as I saw fit, keeping the box for generations of builds. But I started buying prebuilts when it no longer made economical sense to part out a fairly standard setup, and I've totally fallen off the bandwagon. Is there any reason perhaps a prebuilt box wouldn't be a smart idea to upgrade like that? In other words, is it crazy to attempt upgrading the CPU in my current Gateway box?
In order to replace the CPU to something more modern, I'm guessing a new Mobo would be in order as well- but perhaps there are some OEM specific restrictions that an aftermarket board would give me a hard time with? I'm thinking, if I know I can keep this computer current in the near future beyond this upgrade, I wouldn't feel as hesitant spending money on incremental upgrades now.
Or, at that point, would I just be throwing too much money into an upgrade that I would have been better off buying a new prebuilt in the first place?
...man, I need a corporate deal to get boxes like your company does. That would make this a heckuva lot easier.