AN A320 CREW LOSES SPACING FOR LNDG WHEN FOLLOWING THE WRONG ACFT FOR A VISUAL APCH TO RWY 26L AT PHX; AZ.

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APCHING PHX FROM THE NE; WE WERE TOLD BY APCH CTL TO IDENT THE TFC WE WERE TO FOLLOW FOR A VISUAL APCH. SPOTTING THE ACFT THAT I FELT MET THE DESCRIPTION THE CTLR HAD GIVEN; WE CALLED THE ACFT IN SIGHT AND WERE CLRED FOR THE VISUAL APCH RWY 26L TO FOLLOW THE TFC WE HAD IDENTED. THIS WAS NOT THE RWY WE HAD ANTICIPATED AND OUR FMC WAS UPDATED TO REFLECT THE RWY CHANGE. AS WE NEARED THE ARPT; I CONTINUED TO MAINTAIN VISUAL CONTACT WITH THE ACFT AHEAD OF US AND SPACING SEEMED MORE THAN ADEQUATE. THE CTLR THEN TOLD US WE WERE 40 KTS GND SPD FASTER THAN THE ACFT AHEAD OF US. I SLOWED THE ACFT AND SHORTLY THEREAFTER THE CAPT SAW TFC AT OUR L MOVING TO OUR 12 O'CLOCK POS TURNING FINAL AHEAD OF US. HE IMMEDIATELY INQUIRED WITH THE CTLR AND THE CTLR SAID THIS WAS THE ACFT WE WERE TO BE FOLLOWING. SENSING THAT THE SPACING WOULD BE TOO TIGHT; THE CTLR TURNED US 90 DEGS L AND HAD US CLB TO 5000 FT (?) AND HAD US VECTORED FOR ANOTHER APCH WHICH OCCURRED WITHOUT FURTHER INCIDENT. IN THE FUTURE; I WILL TAKE GREATER CARE WHEN CALLING AN ACFT IN SIGHT -- ESPECIALLY IN AN AREA WHERE THERE CAN BE MULTIPLE ACFT. POSITIVE IDENT IS A MUST! ADDITIONALLY; KNOWING THE RWY WHICH WILL ACTUALLY BE ASSIGNED AT THE EARLIEST MOMENT POSSIBLE WILL KEEP AN EXTRA SET OF EYES OUTSIDE THE COCKPIT WHERE THEY ARE OFTEN NEEDED AS OPPOSED TO MAKING AN FMC CHANGE INSIDE THE COCKPIT.