PHP5 has matured a lot over a year mate

PHP 5.2 is incredibly fast and stable, and all good hosting providers have, or offer, upgrades to this version.
PHP 5.3, however, is strongly not recommended at this point in time. A lot of scripts still use code that makes PHP 5.3 fill the page up with error notices (which are like errors, only they don't stop the script from working).
I can of course not guarantee that you don't have some really ancient code on your server that won't work with PHP 5.2, but I'm 99% certain upgrading to 5.2 will be fine