I have my own (small) Xen farm, I typically provision a new VPS from pre-made application templates that I've created for each new site that I put into production.
I have several shared hosting VPS servers that I use for development, or parking sites that get very little to no traffic whatsoever.
Using a few ad hoc scripts, I've made it rather painless to migrate a site from a shared / development server over to more suitable production home.
Keep in mind, that virtualization is handy for other things. I typically use Xen on every 'dedicated' box I have, just as a management layer. It helps avoid needing to rely on on-site hands and eyes to fix most common problems.
Anyway, given the fact that servers are quite cheap to lease, I really recommend getting your own and then provisioning a new VPS as you need one. This also gives you the ability to shuffle resources around as the needs of your sites change.