Hi Raijin,
I would recommend either Debian or CentOS. I honestly, can not recommend 64bit over 32bit; since running an emulator requires ~350mb of RAM. 64bit processes usually require more RAM to run, and you don't even need 2GB of RAM to support 500+ players. So unless you're intending to run a huge server with more than 1k users; then go for 64bit, otherwise I'd recommend 32bit.
Debian as an OS; requires less RAM since it's real barebone. CentOS is like a free version of RHEL; which is an enterprise-grade OS (not free, and not cheap at all), and it has basically almost everything that RHEL has.
Extra Note - Slackware is like Ubuntu... can't recommend it. It install unnecessary things.