With requirements like this, the answer is "anything". No, really. I have a Dell laptop from 2008 that runs Linux Mint on an Intel Core 2 Duo processor and has 8 GB RAM and a terabyte hard drive. It keeps up with my files without breaking a sweat.
In fact, you don't even need a 64-bit processor; you can run current version of Debian or Mint on a 32-bit one...