A 9800pro is MUCH faster than a GF4Ti4600 for 3d stuff. Plus, you get Core Image support, which makes general system usage a whole lot smoother.
However, a 9800 is many years old and nowhere close to "newly introduced" and it is in no way worth $250. You can buy converted PC 9800pros for 1/3 of that price. If you want to spend that much, I suggest looking in to buying a flashed nVidia 7800GS. They offer the best performance out there and should be less than $250.