Yes like William it is said that it is best to upgrade your RAM, since 512 is the standard RAM recommended for any computer. But the reason you are getting those errors is because your computer is performing too many processes at once, and before you used to use your computer you probably didn't install as many programs as you have now (that's why the error messages come after a while after you use your computer).

So if you can get rid of a few processes that you don't need and RAM intensive applications such as Photoshop then you won't have any errors like that. However, the best you can do is get better ram for yourself, and I recommend getting an extra 512 MB ram... that should be more than enough if you don't use RAM intensive processes, and you won't get any errors as well.