Depends on the resolution you are playing at, and what games you want to play. If you are playing at 1080p or lower, you won't see a huge benefit with the 7870 over the 7850 for most games, at least right now. However, if you are playing more demanding titles like Crysis 2, Battlefield 3, Witcher 2, Metro 2033, or want your card to remain competitive at higher graphics settings for longer, then you might want to spend the extra 60 bucks.
I will say, be sure to get the 2GB version of the 7850. There are some games that are exceeding 1GB of video RAM at 1080p resolution. For future titles, you may run into performance problems with the 1GB version of the 7850.