Internet Explorer: It's a browser anyone has used for perhaps years... It's RAM usage is low if you only have 1 tab opened (IE7) but when you have more than that it drasticly rises.
Firefox: A browser that everyone should use. It's a great browser, its faster than IE and more secure. Its RAM usage is high... which kinda sucks for people with older computers/laptops (like me)
Opera: This browser... its REALLY fast its RAM is lower than FF but higher than IE with 1 tab opened.
I suggest getting Opera over FF. Cause its more secure and it has a lot of features. And, there are addons you can make, and download. Although they dont have as much as FF. So if more people use Opera and develop then Opera will rule the browser world.