Having seen the improvements to the graphics subsystem at the WinHec2007 just those alone are worth upgrading for not just for looks but the ability to get some of your hadware back.
Memory Hogs like Windows Aero have been dramatically improved to have 1/3rd the memory footprint And their new Font Rendering brings Windows graphics into the 21st century.
In reply to: "Microsoft exec: Windows 7 is no service pack"
December 3, 2008