Even on a real Mac, Apple always recommended a fresh install to 10.6.X from 10.5.X over an update.
You can try to just do an update if you have a prepped 10.6.X USB thumb drive, but in my experience it resulted in several issues and an unreliable machine overall.
Likewise, using NBI on an older install method (Boot123, DellMiniBoot, etc) caused many issues back when NBI first came out, as I recall. Best not to mix the two on one system.
I seem to remember a post about using a live Linux distro on a USB stick to allow you to recover any data on a non-booting OSX install, but I'm a Linux know nothing.
If you are just wanting a machine booting into OS X and don't care about your current data, your quickest, easiest bet would be a fresh install.