Well, basically speaking, the specs are better, which in turn makes for a 'faster' card.
In this instance, even though the 9600GT has double the memory, the 9800GT is still a 'faster' performing card overall. Benchmark comparison based on fps at 1280x1024 click here=on&prod=on - greater is better. You can do more comparisons with other benchmarks/settings at click here.
There may not seem much difference in them - but as they're virtually the same price, the 9800GT is the better choice in terms of raw performance.
Any particular reason you'll be getting one of these cards? I suppose the answer is quite obvious, but if you're NOT a 'gamer' (&/or you don't have a particularly good spec), you could save a lot by getting something more basic. G