1. It is Officially advised by the Microsoft to go for a Fresh Installation if you are using anything other than Windows Vista SP1. Upgrading to Windows 7 is officially advised for Windows Vista SP1.
2. As you have mentioned that you are planning for 6GB RAM and 64Bit Version of Windows 7 than its OK. As for 64Bit Windows 7 installation minimum RAM requirement is 2GB.
3.Still Windows 7 Developers is working with the compatibility of the drivers with many of the vendors but as far the reports for a 64Bit version Windows Vista Drivers may work. Still Microsoft has online support for about compatibility for only Vista users not for Windows 7. So try it out if it works for you.
Here are some of the useful links which will guide you to the installation process.
1. Use Window 7 Upgrade Adviser to check your system configuration for Minimum System requirements and also checks for the system compatibility. Make sure this tool is a free to download Beta version and you should plug in all the Hardware which you generally use before running this program.
More Information,,,
2.For Software and Driver compatibility go for this official site.
Link
And installation of Windows 7 will hardly take 15 to 20 Minutes.
Hope this text will be helpful for you.