good OC'ed 7950 is on par with a 7970(non ghz), yes a Xfired 7870 outperform it but is it worth the Xfire problems? (scalling, non supported, micro stutter?)
i woul say sell the 7870 get a 7950/7970 and then Xfire it when driver will be better (assumably at the 8xxx launch xD wich means cheaper...