Windows 7, while better than Vista, does need to have, at least, 2 gigabytes of Ram, and a modern processor.
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Microsoft released the final system requirements for windows 7 on April 30, 2009. These system requirements represent the bare minimum required to run windows 7 and are valid for all versions of the OS.
1 GHz processor (32- or 64-bit)
1 GB of RAM (32-bit); 2 GB of RAM (64-bit)
16 GB of available disk space (32-bit); 20 GB of avaiable disk space (64-bit)
DirectX 9 graphics device with WDDM 1.0 or higher driver
If you plan to run windows XP mode with windows 7, the minimum requirements are 2 GB of RAM and 15 GB of additional disk space.
Windows Virtual PC requires a CPU with Intel-VT or AMD-V enabled, as it utilizes the latest hardware virtualization advancements.
These system requirements are basically the same as for Windows Vista. Therefore, your Windows Vista machine will run windows 7 just as well. In this case, you may opt for an operating system upgrade instead of buying a new PC.
According to the Windows 7 Upgrade Advisor, minimum memory needed is 1 gigabyte, running on a minimum 1 GHz processor.