Ugh, I hate how Microsoft incorrectly acts like x86 is 32-bit. x86 is the platform archetecture-64-bit x86 is still x86.
Anyway, whichever matches your OS. Most people have 64-bit Windows. There are several ways to tell the difference, but the easiest is probably to hit start, then start typing "32-bit" and click the result for "see if you have the
32 or 64-bit version of Windows".