If you want to get some worldcup feeling, take the S8 or S9 to downtown Frankfurt and get of at "Hauptwache". From there it is a 5 min walk to the "Roemer" (nice place with old houses) and then another 2 min to walk to the Main River. All the matches can be seen from there
http://www.wm-frankfurt-2006.de/main...&content_id=70
S-Train to downtown FRA runs every 15 mins and it is a 10 min ride.