The 9800 is a little more advanced in terms of the visual effects it can display. The shader programs can be quite long. How that translates into real-world means pretty much nothing currently - and it may be a year before games even support longer shader programs.
They can both produce the same visual effects AFAIK, and both have 128MB ram.
The 9800 is also a faster pixel pusher, and is more or less cutting edge for gamers right now. It can push 50-100% more pixels per sec.
The 9600 is a faster freq. but slower (narrower memory controller) - but 2D for both these cards will be about the same level (very impressive).
That said, according to rumor, the R360 (the R350 is in the 9800) (
http://www.theinquirer.net/?article=10544) has taped out and should hit market in 60 - 90 days. How long it will take to get a Mac version? No clue - but it will probably be after a PC release.
The successor to the 9600 (maybe available mid next year?) will most likely be cheaper than $300, and probably match or exceed the current 9800.
There really is no game which can use the 9800 yet, but that will change over the next year.
-Wyrm