Well it depends TRDCorolla really at what resolution you run games at. Its true, you wouldn't notice much difference at 1024x768 except maybe for the higher frames per second rate, though because it's soo fast, you wouldn't tell anyway.
Its at high resolutions at the moment that'd you'd tell, and with games like Riddick that use so much shaders, and you could probably shove settings up a buit more, and for longer than with the 6600GT in game terms.
If you wanted a lasting card, the 6800GT definetly wouldn't dissapoint,especially sinceif you run at lower res's, you'd notice the enormous boost for so much longer.
As for that card beating the Ti4200...course it would!!!!!
A 6600GT would beat it by alot too, since the new cards run using Direct X-9 which will see more effects in games as well
So yeah, go for the 6800GT if you can afford it
I assume you have PCI-E in that case? Otherwise for that 6800 card, you'll need a PCI-E to AGP swapper. i've heard its good though