I am having EXACTLY this problem and its been going on for a LONG time. (Memory expands constantly up to about 1.4 G, slowing the graphics as it gets closer to that target, then SL crashes.)
I have updated my graphics driver many times. I upgraded recently from XP to Windows 7. I have used the SecondLife official viewer (just downloaded today) and it loses memory slower than Firestorm and Singularity do, but all three viewers have the same issue.
1) Mesh makes the problem worse (I lose large chunks of memory, as noted by Task Manager)
2) Moving my camera in and out eats a chunk of m