It all depends on what you do, 128 is not really enough, especially if you have more than one app open at a time. 512 is a better number and more if you're using apps that require using large files, iMovie, FCP, photoshop, Lightwave, Maya. And then you might need to go to a gig or more.
Memory is really cheap, so there really isn't much of an excuse not to max out.
D