I recently bumped my machine from 512MB to 1GB, to better suit Photoshop's insatiable appetite, but I've still been getting low virtual memory warnings. Turns out, Mozilla is being a pig. Watching it on the Task Manager, I've seen Mozilla consuming 180MB or more. Iconifying it typically causes that number to drop to around 20MB. Bring it back and doing anything jumps it right up to 70 or 80MB again. Weird memory leakage, but at least you can coerce it into cleaning itself up.