SpeedFan has default settings that determine what qualifies as a green check and what doesn't. Since no 2 CPUs or GPUs are alike, I pretty much guarantee you are wrong. For example, my old Pentium D was rated to withstand temps up to 69.9°C (which it regularly hit), whereas my current Core 2 Duo can't take anything over 61 (which it never approaches). My own CPU temp hovers around 30-40°C while playing FFXI, but I haven't checked my GPU. I was getting some freezing--like you, only while playing FFXI, but my screen didn't go black, just whatever was on the screen froze solid. It happens with and without windower. Since replacing my graphics card, it has not happened, but since it's random and the replacement was recent, I don't know if the issue is gone for sure.

Lately I've also been getting the same issue on my old laptop, without Windower. It could be the graphics card overheating--I keep my old laptop at work and usually keep my char in one of the beastman-controlled campaign areas and pound away at fortifications while I'm at work, only looking at the screen every 10-15mins or so, depending on how busy I am at work. As the graphics card fan blows air out the bottom of that laptop, I'm going to try a laptop cooling pad, I'll let you know how it works out. I have gone nights at work with no freezing at all, but they don't happen often, and we're only allowed to have our laptops out for 4 hours, between 2 and 6 AM. If I can go a week (4 working days) without freezing once the thermal pad is in place, I'll send you a PM. If not, assume it doesn't.

short version: you'll have to check your GPU manufacturer's website to find out what kind of temps it can handle. 50°C seems kinda high.