Well I think the most obvious area of improvement (especially if you're talking about going from 512MB to 2GB) is that you can run lots more programs without the system paging to the hard disk and causing slowdowns. You might also look at a program like iDVD, which has trouble even just running a preview of a theme with the stock memory.
Of course, if you didn't use your computer for long with less memory, you might not realize how good you have it with 2GB.