I used to have a 1.4ghz Duron 256MB RAM with a 5900xt Nvidia Video card.
I now have an Athlon XP 3000+ 768MB RAM with the same 5900xt video card.
Both systems played PES4 smoothly at 1280x1024. I would therefore guess that your video card will have the greatest bearing on how well the game runs.
For reference, the 5900xt is roughly equivalent to ATI 9600xt or the Geforce 6200, so as long as you have a semi-decent graphics card you should be ok.