To add some information (which I have to assume TechIan already knows):
I've done some memory profiling on Civ V while playing huge maps (starting, stopping, loading, saving, reloading, etc). I've never seen anything to suggest that a memory leak is occurring.
However, what I have seen is that Civ V uses a decent chunk of memory. It's not uncommon for even FPSes to reach 1GB of memory usage, and strategy games (especially those which track history) take up more. From my own
records, simply starting a game with a huge map takes up ~1.3GB of real (private not virtual) memory.
That is the amount used at the start of the game. As more cities, units, relationships, and history is built up during the game, it's going to need even more memory. I don't have numbers or turn counts to go with this, but I'm pretty certain I've seen Civ V's memory usage creep up to 2GB on my system. As you already know, that's going to be the cap for a 32-bit process.
Now, why did it run fine for me, but not for you? I have no idea. My games have actually been pretty responsive (I don't get huge waits between turns, but I think I know why). Our systems are actually pretty similar. You've got Win7-64 Ultimate, I have Vista-64 Home Premium. You've got a Q6600 (right?) while I have its bigger brother the Q9450. The only big difference is the fact that my video card is much less capable (8800 GTS G92). I even run mine in DX11 mode.
So... The memory amounts that you're seeing aren't out-of-line with what I've measured on my (working) system. So, I'm left grasping at other things: Mods? Background processes? It really does sound like a heat-throttling issue, but those temps are lower than mine (CPU: 50, GPU: 70 ... bet my system is quieter, though).
(By the way: I'm a fellow developer)