The six release versions of Windows Vista have caused quite a bit of confusion, and an upgrade chart released by Microsoft won't do much to clear that up for most consumers -- particularly since Redmond has given XP Home users the widest array of upgrade install options.
For businesses that run XP Pro (and all of them should), the only two options for upgrading atop current files are to Vista Business and Vista Ultimate. (Vista Enterprise, which should be of interest only to shops that are big enough to be on maintenance contracts, is not included on the upgrade chart.)
Vista Business is being touted as most similar to XP Pro, and may be the most logical upgrade path for most companies. A smart piece posted earlier this year at Microsoft Watch suggested that businesses might want to consider Vista Ultimate, which has many enterprise features, as an alternative to Microsoft's Software Assurance program.
Not surprisingly, Microsoft's upgrade path for XP Home users is wide open, enabling them to upgrade their current installations to any of the new common Vista releases. In many ways, this can be seen as an upsell pitch to small businesses that have bought consumer systems with OEM installs of XP Home.
For those businesses, a word of advice: Spend the money. Get on a business-class release of Windows as soon as you can.