I asked this exact same question yesterday. Nobody seems to have any concrete answers. This link--http://eshop.macsales.com/Reviews/Framework.cfm?page=/Tips/macbookcore23gb/macbookcore23gb.html--
provides numbers that show that having a mismatched pair doesn't make much difference when it comes to video performance, but does make a big positive difference when it comes to application performance, like with photoshop. Basically, you lose a few tenths of a FPS while playing Halo/a graphics-intesive game, but your programs/system in general responds much faster.