This may not be helpful, and I can't speak from first hand experience at least wit Quickbooks, but I will give a little info on each. Again, apricot has the firsthand experience.
1.
http://support.quickbooks.intuit.com/support/Vista/ProductCompatibility.aspx#1Most Quickbook products will work with Vista. Many people forget about the ability to set compatibility modes on products. If you right click on a product icon, you can change the compatibility mode and make it run as if it were in a different OS.
2. XP can be faster. However, VISTA can smoke if set up correctly. You MUST have 2 GBs of RAM. If you are going to run Aero, you need a video card that will work with it, but it does take up resources. If using Aero, I would suggest up to 3 GBs of RAM. I just bought 4 GBs for $75 yesterday. It is dirt cheap.
3. I am confused by this one. I have run VISTA as a test machine on my XP Pro network with no issues.
4. Again, I can't speak to this one, but have you tried the latest Vista drivers. It most likely wouldn't work with the XP drivers. Here are the latest drivers for Vista and Brother.
http://www.brother.com/E-ftp/windowsvista/us.htmlHTH Just fuel for the fire.
