I've had the "not enough memory" messages with 256MB. Although I think it's only happened when I've been testing ideas: starting a scenario, playing a turn or two, quitting the battle, starting another scenario, playing 1 or 2 turns, quitting, starting again, etc...
It feels like memory fragmentation - perhaps SP is allocating a large chunk of memory at startup and using pieces of this chunk as required. After a lot of stops and starts, the available pieces might be too small to use. I could imagine this happening if you play a turn from a bunch of different PBEMs in one sitting.
I don't know if this is the problem, but if it is, then completely exiting and restarting SP:WAW (the game, not the battle) every once in a while should help. Maybe after 2 or 3 different battles.
I'll finish this message with the warning that I haven't actually tried this, and I'm probably basing all of this on some flaky reasoning. But the way I look at it is: it might help, and it can't hurt.
