Well, nobody knows for sure, but there are lots of estimates out there. Try to google it to get an as broad spectrum of ideas as possible.
For the absolute minimum of playing without lag in singleplayer, I would say:
2 gigahertz proceccor, 2 gigs of RAM, 256 mb videocard that supports most modern techniques. -such as pixel shader 2.0, DirectX 9.x and higher etc-
But I am no expert at hardware, so you really should ask other people.
