I'm always conscious of trying to keep the memory foot print as low as possible, but a certain amount of memory needs to be available for a web app to work correctly. If this is a hosted environment I'd recommend get better hosting. If this is an environment where you have administrative access to the machine you could investigate further and find out how much memory is available on the machine and how much is being used when this error occurs. If something is spiking up the usage maybe it can be identified and fixed, but if its choking while only using normal amounts of memory there is not much to be done but get more ram.
If you don't use poll, survey, web store, site office you could remove the related dlls from the bin folder. I'm not sure if that will have any impact though because I don't really think they get loaded into memory until they are first referenced, but I'm not 100% sure without looking it up.
Hope it helps,
Joe