To me that's kind of a loaded question. So many things have been added/changed/deprecated in php between versions 5.1 and 5.3. Someone here may be able to pinpoint all the pertainent c5 functionallity effected by php evolution, but that's not one I would want to touch.

My best advice is to load it up, test the heck out of it. If something doesn't work that you need, see if there is a 5.1 PECL module, source module code or workaround for that specific funtionality. Alot of times there are.

Thanks for the reply & advice. I'm new to *any* CMS system (previously used frontpage) & I know no PHP. I only need to build one or two new websites, for my own photography business, & I need a very easy to learn system.

I'm thinking I may have to look elsewhere, either a different CMS or a different server!

most CMS's warn against using 5.1 as it also has huge security flaws. i would find a different host, C5 is the easiest CMS iv used