Vista really isn't worth it yet. Not only are the drivers going to be piss poor for months, but there are going to be litterally thousands of program compatability issues. then you add in the fact that almost all of the "huge improvements" in Vista are just things MS stole from existing third party XP software.
The only real reason to upgrade to Vista will be for DX10 support. However, games will still support DX9 for quite some time and the first DX10 games are quite some time out (6 months or more).
Just stick with XP. I was running Vista for about 2 months and after the novelty wore off, I just reinstalled XP. I would have gone with 2000, but it doesn't support SLI (nor does Vista).
As far as the differences between XPH and XPP, some of the biggest that come to mind are better user control and filesharing on the network. There are a lot more tools in XPP for the tech minded that are sorely missed in Home.