Don't know what you doing, but for my 2.4 SR MBP 4Gig is not enough already 😕😡
Though I had MB sitting along, which had 1Gb, and I still managed to use it 🙁
Now it has 2Gig (from upgraded MBP) and feels much better 🙂
But again it depends a lot on what you doing with your MBook, and on how big a stretch your computing appetites can take 😀🙄