Upgrading to CentOS 5

Before throwing the network online I thought it best to move up to CentOS 5 from 4.5 and get rid of one down the road headache. Acouple questions though:

** Upgrade or new install???

** do virtual server backups (nothing specialized) made on 4.5 restore smoothly after the upgrade or install?

**How is the backward compatability of MySQL and PHP working out?

**Security issues??