For PES 2009, there's nohing to worry about. I got P4 3.0 GHz, 1.5 GB RAM, 6600GT Gfx card and the game runs smooth as silk in 1024*768 with Medium quality options. (You can't select high with that gfx card of mine)

It was all about optimization, though. I was never able to play PES 2008 smoothly.