Windows 7 requirments are listed as follows:
• 1 gigahertz (GHz) or faster 32-bit (x86) or 64-bit (x64) processor
• 1 gigabyte (GB) RAM (32-bit) or 2 GB RAM (64-bit)
• 16 GB available hard disk space (32-bit) or 20 GB (64-bit)
• DirectX 9 graphics device with WDDM 1.0 or higher driver
I would multiply these numbers by two.
Check your system (there were many configurations from Dell), and decide for yourselves. Or, check "Upgrade Advisor" from http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor