It's a pretty decent amount faster. Though I haven't seen any direct comparisons to a 9600GT, I would suspect it's a pretty nice upgrade. I'd say go for it if you have the money. There seems to be some pretty nice deals on there for the 4850 on Newegg.com
But seriously, it seems like a decent card. I need to go test it more. I'm sure that the drivers will be improved soon as well (not that they are bad or anything).
Your current card is more than enough for any source based game at that res and high settings.
So IMO it's not worth it, honestly your card should last you threw this gen. I would say the 4850 is for people like me that have cards like x1900, 7900, 8600gt, 2600xt or lower, those are the people that will benefit most from these.