6850s tend to overclock well enough to match the speed of the 6870. However, overclocking is never a sure thing. If you want a guarantee spend the extra 20-30 bucks for the 6870. Both will be decent fits.
6870 is better and from newegg u can get for $155 after rebate, a 6850 for $135 after rebate, and going any lower like a 7770 for $105 is more just a money saving solution. if u have the money and want better performance I'd say go 6870 either XFX Double D or Sapphire.
Not sure if it'd cause a bottleneck with the FX-4100, but a Sapphire Radeon HD 6950 u can get for $180 after rebate and would b better than the 6850. It comes in 2gb version for higher resolution/multi monitor which the 6870/6850 r 1gb versions and the one 2gb 6870 is similar priced to the 6950.