I'd go with the 4870... 2 reasons
1.) In cf, adding a 4850 would slow your 4870 down to act almost like a 4850.. load balancing.
2.) In time, if the 4870 gets a bit slow for your requirements, a second will be cheaper then what you bought the first one for. Additionally, adding a 4870x2 may be a very viable option for a tri crossfirex setup. In a years time, they could also be quite cheap.
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