As others have mentioned, a mid-range machine isn't easy to define, but I generally consider mid-range as something that was state of the art two years ago.
Of course an excellent tool is Valve's hardware survey:
http://store.steampowered.com/hwsurvey
I don't have time to calculate it exactly, but it looks like the average gamer's hardware is between 2-3 years old. If we can run PS2 on "average" machines, I think it will be an incredible success.