Whilst flashing a new ROM will wipe your phone, it doesn't generally touch stuff on the SD Card so that will be safe. If you want to be doubly safe, there's no harm in backing stuff up to the PC anyway.
When you say you did a nandroid Optimus V backup through ROM Manager, what exactly does that mean? A Nandroid backup is a snapshot of your phone at the time the phone was backed up, to be applied if anything goes wrong with a ROM flash. If it's an Optimus V backup, I suspect it wouldn't play nicely with your Pheonix (it might even brick it).