A 64-bit machine can access much more memory and run bigger applications. Speed may be faster, or slower, depending upon a lot of factors too complex to go into here. If your system has less than 4GB of RAM, go 32-bit. If 4GB or more, go 64-bit. FWIW, most 64-bit systems can run 32-bit apps just fine, and contrary to what jingda said, they shouldn't crash, unless you are missing some 32-bit shared libraries (not uncommon, but not hard to fix). I run 32-bit apps on my 64-bit systems all the time. I had trouble with 32-bit Skype because of the missing shared library issues, and it took me an hour to figure out what they all were, and install them. Once done, Skype works perfectly on my RHEL 6 system.