Unless you need to access more than 3GB of memory (and have it installed on the machine), or you have a specific need for 64-bit (i.e. need to use an app that only has 64-bit binaries), you should use the 32-bit version. I use the 64-bit version on a 2GB machine, but I only run server software on it, and I'm building software that needs to build against the 64-bit compiler. Generally the AMD64 architecture has slightly faster binaries because it has more registers which the ABI supports. This means less pushing onto the stack which is slow (compared to registers). Some people argue that the wider word size causes more cache misses which makes up for this, but I'm skeptical of that claim.