MySQL on Ubuntu 32bits vs 64bits
In 2014, using 32bit architecture for any critical app is insane. So, I would suggest: 


Upgrade your web-app server to 64bit (soon you will not find 32bit
editions for most software). It is not that hard and you get better memory management, performance with the exact same hardware. 
Still, if you do not want to do that, install MySQL DB (64bit edition) on a 64bit Ubuntu server to get at least there the benefits of a 64bit architecture. There is no danger when you interact with SQL queries if the architectures of the web-app server and the DB server are different. When (when and not if because it will happen eventually) you will be forced to change your web app server architecture, you will have one less thing to worry about.