To my grave technological embarrassment, I just spent the afternoon using the old Thinkpad SL400 to surf the web, look up Linux stuff, check out some movies, download utilities, have torrents seeding in the background, and etc, all on a 22" external monitor, and ye olden machine never broke a sweat. I had a temperature monitor running the whole time and it rarely hit even 80C inside the box. If 80 degrees Celsius seems high, for this computer it's not. It didn't even trip the fans once. This is the same old laptop that three days ago got so hot it froze after only five minutes running the external monitor. The difference? A couple of days ago I finally got up the nerve to remove the fan assembly. Once it was out, I was able take a makeup brush to the vents and puff away a few too many years worth of accumulated dust (and coincidentally bang a lot of dust out through the keyboard).
I still have peeves about Linux - screen dpi, mouse wheel settings, weird window handling, goddamn LibreOffice, the fact that Windows 7 just is less eye-aching on the very same monitor - but I might put an ssd in that old SL400 and see what happens then.