Go for the 9800Pro all day long it really does offer twice the performance of the 9600XT.
9600XT 128bit RAM 4 Pixel Piplines
9800PRO 256bit RAM 8 Pixel Piplines
The Pixel Pipelines and very important as it is the cards speed at which it can fill the screen with the image, considering the X800XT and 6800 GT/ULTRA'S have 16 ! You will see that the 9600XT is really old technology.
There is no sound argument if both cards are in your price range that puts the 9600XT first , other than you dont want a card that need to be connect to you PSU, which is very easy to do.
Just type in 9800Pro vs 9600XT in google and it will confirm this. GET THE 9800Pro.
Better still get a 6600GT 128MB @ £144.99 Which hammers the 9800Pro :)
Or even better branch out and get a 6800 GT 256MB like me! ( just kidding :) the 9800Pro is a great card and if it's in your price range get it!)