I have a X100e Thinkpad and it came with 3GB of ram and a 32bit operating system and I was hoping to upgrade it to 4GB, but I've read it is not worth doing so unless it's 64bit. Is this the case?
Conventional wisdom is that 32 bit Operating systems will not be able to make use of much memory beyond 3 GB for user applications since some is reserved for video, and there are some differences in system optimization.
One approach when considering whether the upgrade is worthwhile, especially if it means updating to a 64 bit version of the OS is to determine how much of the existing system memory is being used.
This can be checked this by pressing Ctrl + Alt + Del within windows to pull up task manager, and then choose the performance tab, to see what percent of your ram is being used. In this XP version below, less than 1GB is being used.
There are a number of good reference sources to learn more about 32 bit addressability and the theoretical and practical limitations of system and OS configs.