Depends on how good you want it to be. A great gaming computer could very easily cost you thousands of dollars. An ideal setup would be Alienware, with a liquid coolant, but that's a bit over the top for Sims.
For Sims, I would consider an Intel, Radeon or Nvidia graphics card (not sure about that, I'm not very hardware-oriented), with 1TB of memory and a Solid State Drive, with
at least 8GB RAM.
Based on things I've heard, if at all possible, avoid Windows 8, it's not very gamer friendly. Stick to Windows 7, or a Mac. Other operating systems such as Linux are not supported by Sims, if I recall correctly.
There is a thread
here called Ultimate Computer Specs, which should give you a decent indication of the sort of thing you want.
I have a laptop better suited to work-type stuff mostly (ie, Word Processing, Spreadsheets, etc), but it runs Sims fairly well with a Radeon graphics card and an Intel Pentium processor. It's an HP Pavilion g6.