I have no idea what causes Kirsten's client, Firestorm, or the 2.7.x beta to decide to grab all available RAM and crash.
Just now I fired up Firestorm twice; both times, it sucked down 3 GB of the 4 GB available and crashed--OK, the first time I saw it coming and signed off before the inevitable. Rule of Three: I started it a third time, and it behaved. No change in settings; I didn't clear the cache.
Maybe it will settle down into mostly working, the way the previous Firestorm did. I hope so. I may try global illumination again; if this is a Heisenbug, I wouldn't be surprised if it was just chance that made me think it affected anything.
I wish there were a real 64-bit build of Firestorm that I could try.
UPDATE: I tried global illumination; no apparent difference. It just happened to crash once when I had it on, and then not crash the next time when I happened to turn it off.