No, Safari doesn't need (or use) 500 MB or anything close. That's why adding more RAM isn't going to fix the tab reloading problem:
It's not due to lack of RAM.
I don't know why people assume that it is, and cling to that idea against any argument. (Not saying you do, but there are some people that do. I think you're on the right track when you question if Safari could possible need 500 MB per tab... Of course the answer is no.)
If it is not a lack of RAM, then why does Safari always re-load its tabs? I can't think of another reason. It is incredibly annoying (especially when you are on slow internet). One time, I was filling out a form and needed to look something up. I went to Wikipedia, found what I needed to know, and when I went back to my previous tab, Safari had already clobbered it losing everything that I had typed.