Microsoft have released the Windows Vista Upgrade Advisor to analyzes your system hardware and help user if their computer could handle the requirements as well as find areas where they could improve.
The official requirement for Windows 7:
* 1 GHz or faster 32-bit (x86) or 64-bit (x64) processor
* 1GB RAM (32-bit) / 2GB RAM (64-bit)
* 16GB available disk space (32-bit) / 20GB (64-bit)
* DirectX® 9 graphics processor with WDDM (Windows Display Driver Model) 1.0 or higher driver
Download: Windows 7 Upgrade Advisor beta