<p>We want migrate from Delphi 7.0 to Delphi 2009, to stay up with a modern IDE and current technology. Our target platform is .NET. What is best way, to achieve this goal?</p>

Migrating from Delphi 7 to Delphi 2009 is probably straight forward, there were only minor changes (such as the recently discussed move from ANSI to Unicode strings).
The move to .NET is not so straight forward, though. Although there's a VCL.NET which makes migrating GUIs possible, severeal other things will not work. So you might think of having Win32 code AND .NET code, bringing them together through COM.
(I hope you already know COM, otherwise this is probably not an option;-))