You don't have to upgrade to Windows Vista to get the really cool U.I. effects. You can transform your old Windows XP with a couple of FREE tools.
Transparent Window Borders
To get transparent window borders, you can download TrueTransparency. It's got 4 different 'Aero'-styles and it works on Windows Vista Basic too!
You can get thumbnails of windows that you have open using Visual Task Tips and that's a free download too. However, the difference from Windows Vista is that this one doesn't work if your window is minimized but it works just fine with background windows but, hey it's free so that's good enough for most of us.
3D Window Switch
The coolest effect by far is the 3D window switch in Windows Vista when you press Windows+Tab. Now you can get it in Windows XP (or Vista Basic) too by downloading a little program called WinFlip. WinFlip also labels your windows in the 3D switch screen so it's actually easier to use than Windows Vista's flavor.