The Mac Pro is the only machine where as many components as you've mentioned can be upgraded. As Nick said, Macs aren't gaming machines (though they can do gaming pretty well) and aren't designed around that priority. A friend of mine is currently reviewing a $3K Origin gaming PC, but has already said that if you really want to trick it out for that purpose, you're looking at $10K. And if you really want to use it for 3D rendering or high-end video, you're looking at $25K. For a computer.
For gaming and with a $700 budget, about your only hope is a used prev-gen Mac Pro, which should still be a decent gaming machine with a decent card in it. The processors in the pre-2013 Mac Pros are not upgradeable to the best of my knowledge (possibly in some models), but the GPU is (and of course RAM, to a point).
I think you'll find that once you have a job you'll have a lot less time for gaming -- and this is not necessarily a bad thing. I used to play a lot of games, but now I much prefer being **incredibly selective** on which games I will spend my precious free time with, and this has made the actual gaming **experience** more fun. The situation has also "forced" me to spend less time in fantasy worlds and more time in the real world, which I believe has also been a big benefit. We all enjoy the occasional escape, but too much time in gaming, I find, has negative consequences for both health and well-being.
