for standard image quality, the GF4 beats the GF2.
as for tv-out quailty, i wouldnt think there was much difference as practically the same tv-out chips.(dont quote me on this)
as for the upgrade from a GF2 - GF4 Ti4200, go for it. the GF4 is far faster and will allow you to play almost every game at max detail. and the 2d in windows is better.
if your into overclocking, the albertron "medusa" Ti4200 is amazing for overclocking, you can overclock it to Ti4600 speeds easyly. dunno if theres a 128mb version of it though.
IMHO, if you go for a GF4 Ti4200, make sure you get the 128mb version. then overclock it to 4400 speeds (or beyond). just make sure that you research into several reviews of the card you get as it can tell you a lot. like whats in the box, how it overclocks ect;