i686 is what you're looking for in that case. Put short, the 64-bit versions are usually marked with x86_64 and anything else then is 32-bit.
EDIT: The iNNN refers to the machine type the package/disc is meant for; i386 refers to the Intel 80386 chip, and i586, i686 etc. then refer to newer models. Anyway the discs marked with those are meant for 32-bit systems, whereas those meant for 64-bit systems are marked differently (they should always contain the number "64" so you'll know what it is for).
Last edited by b0uncer; 09-16-2008 at 06:00 AM.