Today and tomorrow - July 10 and 11 are your last days to pre-order Windows 7 at Microsoft's special price of $49 for Home Premium Upgrade and $99 for Professional Upgrade. You order today or tomorrow but don't pay until the operating system is shipped on October 22.
Microsoft is also offering free upgrades to Windows 7 from Windows Vista for any new PC you buy from now until October.
The system requirements are not as high as for Windows Vista. It reportedly runs great on netbooks. But you still might want to run the Windows 7 Upgrade Advisor to make sure your PC will be able to run the new OS.