6586
It always makes sense to update to the latest versions, as they are the most secure. What version do you have now?
But your problem is probably with the modules. So once you login, deactivate all modules and see if it works. If it does, try to install one by one, and see if the homepage is OK. If it becomes blank, then that's the module which is causing it, and you would have to make sure that you either download the latest version that hopefully is PHP5 compatible, or you'll have to fix the module yourself.
BTW - there could be more than 1 module that is not PHP5 compatible, and causing problems.