There are tons of reasons to upgrade, and that’s out of scope here.
I’m running 4.7 x64, and I want to upgrade. After googling around, found out that I have to upgrade by grabbing packages from the centosplus repo.
Consider reading http://wiki.centos.org/AdditionalResources/Repositories/CentOSPlus/CentOSWebStack?action=show&redirect=Repositories%2FCentOSPlus%2FCentOSWebStack.
At first try, php-pear won’t let me pass the Yum transaction check, and after googling for a long time. I gave up. Screw php-pear, I rpm removed it with –nodeps.
Then Yum would go through, but soon met lots of MySQL related file conflicts. Again, after googling, remove MySQL i386 version if you have it.
After that, just run a yum update php mysql (must enable centosplus repo) 
I have recently upgraded my system to PHP 4 and 5, and after upgrading several Drupal and Joomla websites only experienced once/twice blank pages and soon become good again. However, Wordpress kept on giving me blanks.
After some googling, I traced to 2 causes:
1. I traced the program steps and found the problem to be in wp-includes/taxonomy.php file, the following line is causing this:
$wp_taxonomies['link_category'] = (object) array('name' => 'link_category', 'object_type' => 'link', 'hierarchical' => false);
The author here suggested to put a new function to transform object manually, which solved his problem. However, after some more googling, solution #2 popped out.
2. The author here found that it’s the problem with PHP 5 setting zend.ze1_compatibility_mode, if one turns this off or put an off flag in .htaccess, the problem is supposed to be solved.
Now, #2 worked great and I think that’s the right root cause here without having to hack the Wordpress files 
