Answer Basically what you have now and maybe some more RAM.
As for Vista it's OK for Home Users maybe but I personally hate being a Paying Beta Tester which is what the current Home Users are.
When it comes to business systems it's currently not suitable as there are still numerous Applications that are not supported issues with drivers suitable for Vista and problems with HDD formatted on different OS that are unreadable. All of that currently makes the Vista Offerings unsuitable for Business currently.
Not to mention problems with Networking that require radical different procedures to allow them to work nearly as well as XP currently is doing.
Personally I'll leave the Home Users to test it and work the bugs out before I even consider rolling it out in a Business environment. It's really great to have such a vast pool of [b]Beta Testers[/b] out there to fix the problems that have been created by the coding. :D
BTW I've had the production version of Vista Ultimate since November 2006 and I'm still to permanently install any of the licenses that I have. Maybe when the Quad Cores become more common in Desktops.
Col