64 is neither better or worse (actually it's worse for php) ... if you are getting 3GB + then you have no choice but going 64bit .. if less (most likely) then 32 bit is better because most php libraries via yum/apt are compiled 32 bit.
I found that some extensions do not even compile under 64 bit distros ... and I have found that APC slams will only occur on 64 bits (None under 32 bits this is my own experience)
@Nasr hahaha APC slam will give your favorite error GATEWAY ERROR
If you have anything less than 512M RAM, use 32 bit. I have one 32bit 512RAM VPS, and a 64bit 1024 meg RAM VPS that I use for a database server.
I started out on slicehost with a 256 "slice" and 64bit CentOS. A whole lot of swappin' was going on.
One problem I had when I was running 64bit CentOS, is that there due to some yum bug or something, it installed a number of 32bit packages in addition to the 64bit ones, which caused some problems with other repos I had installed.
If you go 64bit with CentOS, make sure to check and remove any 32bit packages that are also installed, although I think and hope that bug is resolved.