Well I've found performance of Ubuntu 13.04 64 bit on low end pc's to be very sluggish. My 3 years old laptop came installed with Windows 7 64 bit, but only has 3GB RAM. In the past I used the 32 bit version of Ubuntu because that was recommended, but since 13.04 I moved to 64 bit and everything was so much slower even on a clean system. The laptop was pretty close to overheating all the time. After a few months I decided I've had enough and installed 13.04 32 bit and it's back as smooth as 12.04 32 bit (what i'd been using before 13.04 64 bit).
So I'm suggesting: even if you have a 64 bit cpu, if your amount of RAM is less than 4GB, I'd still recommend the 32 bit version.
@sudbash: I know for a fact that you need to create a 32-bit wineprefix to install the dependencies for MS Office (2010) on a 64 bit machine. It's not impossible, but there's more hassle with the installation on 64 bit than there is on 32 bit. Installing Office with Wine on Ubuntu 32 bit is almost as easy as installing it in Windows, and it isn't with Ubuntu 64 bit.