Linda,
You are at NetBackup Enterprise Server 5.0 and hopefully at some sort of Maintanece pack version (MP1, MP2, MP3 and or MP4), the next step is to upgrade to NetBackup Enterprise Server 6.0, < -- Version 6 is a bit more of a resource hog, with that aside if I were in your shoes I would stay at Version 5.0 with the latest MP and bring the new servers into you enviroment up to the same version. When the Server project is done and things have settled down then I would consider upgrading to NetBackup Enterprise Server 6.0.
JD