A Radeon 7770 would match your CPU more or less, but will still be pretty slow.
Here are the minimum specs you need
Windows Vista, Windows 7 or Windows 8
• DirectX 11 graphics card with 1Gb Video RAM
• Dual core CPU
• 2GB Memory (3GB on Vista)
8400 won't run it, it's not a DX11 part.
Get a Radeon 7770 (as long as your power supply and case can run it), need 4gb system RAM, and then you should be able to run Crysis 3 on lower settings.