U can do the comparison right <A HREF="http://apps.ati.com/ATIcompare/" target="_new">here</A>.
9800 has an advantage of having 256 bit memory interface (9600 has 128 bit) but 9800 (non XT) clock speed is less than 9600XT. See the comparison chart for more details.
As for real world performance, look at the <A HREF="http://forumz.tomshardware.com/hardware/modules.php?name=Forums&file=viewtopic&p=409019#409019" target="_new">thread</A> that relates to Doom 3 performance results. A lot of us have posted out results there and U can see the performances of the cards. In the end its u have to decide how much are u willing to pay. pounds 140 ~ $200...$240 can get u a good card.
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<font color=blue>Futile is resistance,</font color=blue><font color=red> assimilate you we will.</font color=red>
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<b>Hard work has a future payoff. Laziness pays off now.</b>