Frank_Rizzo - 10:40 am on Dec 7, 2011 (gmt 0)
When I moved to 64 bit a couple of years ago I ran a few tests to see which was faster.
32-bit passed all tests as the fastests but obviously there is the 4Gb limit - which is going to be useless if you are running a meaty DB.
You want the O/S and apps to use the RAM for caching, buffers and stuff. Thus for any rig with > 4Gb you are better off with 64-bit.
One alternative is 32-bit in PAE mode. This allows 32-bit to break the 4Gb barrier. But there is a slight performance hit with this.
As the others have said you are better off doing a fresh install with 64-bit version.
One gotcha with Centos 64-bit is that you can get into a mess with it installing some 32-bit apps and some 64-bit. In yum.conf put
exclude = *.i?86
right at the start, before you do any further yum install / yum updates