Short answer use 32-bit if you have 3G of ram or under. If you have 4G of ram or over use 32-bit with PAE (kernel-PAE).
Currently adobe flash isn't 64-bit on linux. (only beta drivers exist) The gain you'd get for 64-bit kernel just isn't worth the hassle if your not an expert. PAE can support up to 64 G of ram so your good, just run 32-bit.
Ref: http://en.wikipedia.org/wiki/Physical_Address_Extension#Linux
