Plugins and such have moved on now especially the 3rd party ones in terms of performance and cpu usage. I'm in the same boat and just about to replace my macbook late 2006 with 4gb ram as it just wont cut it anymore once i start using a few sample instruments and plugins along with audio.
plump for the i5 if you cannot afford the i7 just so you'll get more life out of the computer. refurb store is a great place to save money on models by the looks of it.