I have 512 MB as well but never get those warnings. Check your swap file in windows and your HD space -- make sure the swap file isn't too small (or too large) and that you have plenty of HD space.
Also close running programs. I can have film viewer open, photoshop open, MSIE open and be doing all the stuff needed to find a screenshot, take it, paste it, save it, and upload it, and I never get that error.