Just installed a matched pair of 2 x 1GB DDR2 PC4200 in my 1.83 Ghz Macbook, and the performance difference is negligible. Mind you, web surfing, email, iTunes, aMSN, iPhoto isn't really anything intensive; but I can barely sense the increased performance in OS X, if any. I can open A LOT more tabs in Firefox without a single sign of hiccup, so even just that makes it a good upgrade for me. Unless you game and/or do a lot CPU intensive work like iDVD, or iMovie, 1.25GB of ram is more than enough. The jump from 512MB to 1.25GB is definitely a MUST DO. But 1.25GB to 2GB is not as significant, the only way you would know you have 2GB of ram is if you look in "About This Mac". EDIT June 25: I exchanged the ram for a correct pair of 2 x 1GB DDR2 PC5300, and still barely feeling any real noticeable difference.