I'm going to be buying the new 15" mac book pro soon, and I was wondering, will i notice a big difference in the 2.53GHz MBP, over the 2.4GHz. If I were to get the base model, I would upgrade to 4GB of ram, the 2.53 already has 4GB, a bigger HDD, and some other upgrades. Is it worth the extra $600.00? Or should I get the base model, with the 4GB?, and maybe buy an external drive or an extra battery. any suggestions?