I am about to download the latest version of Eclipse IDE, Helios. Naturally there are packages for various types of platforms. I know I have a Mac, however I also had to choose between 32 bit version and 64 bit version. There was a time I exactly knew these kind of things, but then I also used to build my computer myself.
Fortunately, there's a Apple support page which exactly tells me exactly that: How to tell if your Intel-based Mac has a 32-bit or 64-bit processor