Dont expect football games to be as demanding as other next gen games such as say, Crysis or Halo 3, etc.
They havent pushed the hardware to the limits yet, so your average high-end PC should do just fine. By that I mean, 2-3 Giggs(lol, couldnt resist) of ram, 2.5 GHz dual core processor and something that's at least as good as Nvidia 7700 ( I bought mine 2 years ago, now it's less than $100).
All in all, footie games have never been graphically intensive. That being said, 2010 versions will see a marked improvement.
I just hope they improve the crappy 2-d crowd in PES. Those guys are hilarious.