If you don't include the graphics card, the most demanding game has got to be Championship Manager (soccer management game). The latest version which is out in November has a database of over 40000 real-life players and staff. You can run up to 26 leagues at once all with their own independent league rules, transfer windows. All the leagues are fully interactive with each other including things like media rumours, player and team stats, international competitions, and the ability to view other ongoing matches as you play your own. One save game (compressed) can be anywhere from 500MB to over 1GB containing up to 20 years of historical data from your career and the teams you manage. Running one season on the version I have now, which only has 16 leagues, on a P3 700E can take up to one day (about 12 hours) of continuous (and I do mean continuous) play.
Just a shame there are no real graphics only windows style drop down menus and background pictures.