are you buying two radeon hd 7850 from the begining or adding another one to an existing 7850? what are your full specs and budget?
either way, you should wait till amd fixes eyefinity issues with 7000 series cards (crossfire) as well as for directx 9 games. afaik, they haven't yet. they only mitigated (i.e. started to fix) single monitor frame pacing issue and single card stuttering.