As far as making PS2 look better on an HDTV you can buy an external scaler but those cost big bucks, more than an HD system would.
Gamecube graphically speaking is superior to the PS2 by a good deal, with high quality Component cables GC will look better in general terms than a PS2 could. However the problem is PS2 with more support developers found ways to work around it's being a weaker system overall so it's possible some games could look nicer. Component cables or external scaler are basically what you can do.







