What most people here seem to be forgetting is that with the 2.53Ghz model, you get a bigger HD, 4GB of RAM, and double the CPU cache. Is it worth it? Well, in your particular situation, probably not.
That is, once you factor in the restocking fee, it would simply cost too much. Like most have said, you're better off upgrading the RAM and HD yourself - I doubt the CPU speed/cache will make a substantial difference.
Hope that helps!