Was that what Process Monitor caught? The Sims 3 crashes between 1.6 and 1.7 GiB. Maybe it has to do with how much memory is allocated to the video card. I see you have a 1 GiB card and since you are increasing the texture sizes, it is possible that the game could be reserving 800 MiB to the video card while TS3 reserves around 300 MiB. VRAM is really the only possible explaination why it isn't always 1.99999 GiB before it crashes.