Well.. if the 9850 is free why not see if it works for you then spend the extra cash when you need to?
Certainly it is an inferior CPU compared to the others, but teh price is right. What is stoppign you from trying teh 9850 first, then upgrading (prices usually dont go up for the holidays) if it turns out to be bad for you?
Firstly, i you are goin to try and overclock then go for the phenom II as the phenom 9850 is a dog for overclocking even with the unlocked multi. 2.9GHz was as far as i got and had to buy aftermarket cooler coz of the temps! 47*c idle and over 61*c on load...61 is the threshhold so didnt let it get above that but would have quite easily!!
But considering its free its not bad at all...It will probs have plenty of bandwith for your 5850 and likely not bottleneck it and considering DX11 improves Multi core support with graphics it will even less likely bottleneck i presume?
Edit: Forgot to say i saw no improvements in FPS going from stock (2.5GHz) to 2.9GHz on CS:S stress test; indicating no bottleneck. But thats with 2x8800's.
Less caps lock please. And yes it is worth the upgrade if you're experiencing problems. My recommendation, buy the 5850 if you're unsatisfied in your current 3870 for gaming. It should be a vast improvement, even bottle necked. Then upgrade the CPU later if you aren't satisfied with your system.