I guess at some point in time (unless you use Linux as an operating system) most people are going to have to upgrade to Windows Vista. For most people that means a switch from Windows XP. A system running Windows XP may not necessarily be able to cope with the top of the line edition of Vista - or not all of the graphical features anyway. The minimum requirements for Vista are higher than for Windows XP. But most systems can be upgraded relatively painlessly to cope.
Microsoft offer a Vista Upgrade Advisor tool.
This basic guide runs you through the process of using the guide and understanding the results so that you can make an informed decision about upgrading to Windows Vista.