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The crowds have thinned a bit and now you can pick up a copy of Windows Vista at the closest retail store (or online). But on the shelf is a selection of Vista flavors, ranging from the Home Basic and Home Premium to a Business edition and Ultimate edition (plus one for 64-bit computer systems).
Microsoft offers a Windows Vista Upgrade Advisor to help you determine what your PC can handle. Or, you might start by installing one version, get a little better acquainted with Windows Vista, and then look to upgrade to a different edition. Oh, and what about the other PCs in your household? Maybe one gets the upgrade first, and later you'll look for the appropriate edition of Vista for the other systems. You have several options.
How to Trade Up

If I choose and buy one version of Windows Vista, can I upgrade to a different one later?
Yes. Microsoft offers a Control Panel applet that will let you upgrade to a more feature-rich edition of Windows Vista. The process is called the Windows Anytime Upgrade, and you can order the upgrade from Microsoft's Vista site.
If you start with the $199 Home Basic version of Windows Vista, you can upgrade to Home Premium for $79 or to Ultimate for $199. From Home Premium, you can upgrade to Ultimate for $159; and the upgrade from Business to Ultimate costs $139.
What's more, volume purchase prices are available not only for offices, but for the family. If you buy a boxed retail product of Windows Vista Ultimate for a PC in your home, you can get an upgrade license to upgrade one or two additional home PCs to Windows Vista Home Premium (if those other PCs are currently running Windows XP, 2000 Pro, or Vista). The price for this home license is $49.99 per upgrade license, a 60 percent discount over estimated retail prices.
Installation Alternatives

Can I install Windows Vista on a system that doesn't have a DVD drive?
Yes, but you'll need replacement installation CDs from Microsoft. You can request the 32-bit version of the operating system through the Windows Vista After-market fulfillment program. (The 64-bit version of Windows Vista is available only in DVD format.) Inside the retail box you'll find directions to a Website with information about the fulfillment offer, and a link to the online order process. Customers must provide their product activation key (found on the DVD case in the retail package), shipping address, and billing information. Microsoft charges a nominal--unspecified--fee to ship a CD containing Windows Vista.
Can I run Windows Vista on a Mac?
Yes. The two major options for running Windows on a Macintosh system--Parallels and Apple's Boot Camp--both support it.
I've heard all kinds of horror stories about how the Windows Vista license will force users to buy another copy of the OS if they upgrade their PC. Are they true?
No. Initially, Microsoft imposed some pretty severe restrictions on Windows Vista usage following computer upgrades. But the company backed off after an outcry. Windows Vista licensing is now identical to Windows XP licensing: You can transfer your license to a new or rebuilt PC as often as you like, as long as you uninstall it on your old machine first.
Next: The role of Windows XP in a Vista world.

Source:PC World - Fire Up Your Vista Upgrade