The basic requirements for Windows 7 are as follows:
1. 1 GHz of computing power, either 32-bit or 64-bit would do
2. 1 GB of memory for 32-bit and 2 GB of memory for 64-bit
3. 16 GB available hard disk space for 32-bit and 20GB for 64-bit
4. DirectX 9 graphics device with WDDM 1.0 or higher driver
The features above is enough to run the basic functionalities of Windows 7.
Other features you want to use for Windows 7 requires additional resources across memory, hard disk space, graphics power, etc.
Post from: Windows Vista Weblog