There was a full moon last night. Maybe that's why Firestorm worked for me last night and this morning, delivering a consistent, if not astounding, 10 or so fps at ultra graphics settings without laying waste to available RAM.
I'll fight off the urge to not change anything lest things go to heck in a handbasket again, and try other clients. How could this happen if it's my computer? OTOH, it's had to believe a server issue would make clients allocate huge amounts of memory. It is, as a fictionalized King of Siam would say, a puzzlement.
UPDATE: Cleared cache on, and started up, Kirsten's client's latest build. It took a while to pull down the textures and information on surrounding structures, after which I started walking around. It was reasonably responsive for a minute or two, and then it took the plunge. The system monitor showed it allocating RAM at a rate of about 100 MB every twenty seconds, 1.3 GB and climbing. I signed off.
UPDATE: Tried Firestorm again... and it worked. On for what seemed like forever compared to recent fiascos (which probably means it was maybe fifteen or twenty minutes), and the most RAM it took up was a bit over 800 MB. I even fired up the System Monitor and saw its RAM consumption decrease occasionally! Res miranda... oops, wrong civilization. This was with graphics cranked, including, once I found the knob on Firestorm, global illumination. I think I've made my choice.