Use x86, hands down. I installed Fedora x86_64 on my ASUS eee 1001p with 1G of RAM and that was a mistake. In hindsight, it makes complete sense: 64-bit addressing makes programs take up lots more space in memory (because the pointers and stored memory addresses are twice as large) which makes systems with not much RAM not work as well. When I reverted back to an x86 Fedora, WOW! Things went much faster. There was oodles more memory available for applications.
Lesson Learned: Don't use x86_64 unless you absolutely need access to more than 4G of memory. And on my ASUS 1001p laptop, that'll never happen (system can only handle 2G I think) so it just doesn't make sense.