64Bit OS primary advantage is being able to address more RAM natively.
x86 or i386 architecture can only address 3.2GB natively, afterwards it uses PAE ( physical address extension ) to map the rest
So if you are using 4+GB of RAM then go with 64Bit -- otherwise it wont make much of a difference as most server tasks like Apache/MySQL wont be heavily influenced by 32vs64bit.
As a standard rule we've switched to installing 64bit OS's whenever possible as its impossible to upgrade from 1 to the other without a format.
The only downside is its a bit tricky to get a few apps running in 64bit OS - the upside its honestly a simple process - get a 32bit machine, compile it there statically and copy it over
But when you have only 1 server - it can be complicated