My case only has openings for 80mm fans, of which I have three (one intake in the front, one exhaust in the back, and the PSU fan, as well as a system blower that occupies the vacant slot above my gfx card). Would it be okay if (after removing everything from the case obviously) to drill holes to accommodate a 120mm fan in the back or would the vent opening still be too small?
OR is this just something I shouldn't bother with because it wouldn't make any noticeable difference and would be a waste of time? (Please don't suggest getting a new case that has larger fan openings, I'm far too poor )

I havent tried this personally, but it probably isnt going to be worth the effort. It would be better just to buy a higher airflow 8cm fan until you can afford a new case. they arent that expensive, mine was only $85

$85? That's expensive if you're in my price range, I'm more of an ebay kind of person, the only reason I ask is because I have a few 120s sitting around and I figured I'd see if I could do this, my PC runs at a constant 25c (according to speedfan) so I'm not really desperate for more airflow through the case...just curious and figured if I could do it for free might as well...

If that is what your running, then its definetly not worth it. if you were running dangerously hot, i would say maybe. But at those temps, dont even bother.