With anything below a Mac Pro, I think it doesn't make real sense to go 64bit, anyway. The big step in 64bit computing is that you can address more memory. You know: The amount anything below the Mac Pro can't take, anyway. The 32bit limit is 4 GB. The iMac's limit is 4 GB. MacBook Pro: 3 GB. MacBook: 2 GB. With your 1.5 GB, you shouldn't really have to care about 32bit/64bit. Even better: It's seamless, anyway.