As Reece said, it's memory related. On XP 32bit there are options to squeeze more out of your memory for the bigger mods but ultimately you will hit a brick wall. This issue used to drive me crazy until I set up my OS with a /3GB switch, patched my .exe with a 4GB patch and started the game only after a full restart and use of gamebooster. I also had to ditch the FMNewinterior mod as that was just too much for my already heavily modded game.
W7 64 is but a mere few months away though when i upgrade my system.
