B737 CREW TUNES RADIO TO WRONG FREQ ON APCH TO ORD.

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ORD APCH HAD CLRED US FOR THE RWY 4R ILS; BUT MAINTAIN 250 KIAS UNTIL FURTHER ADVISED. THEY ALSO TOLD US TO CONTACT TWR 120.75 AT REKKS. I WAS HAND FLYING THE APCH (THE WX AT ORD WAS ABOUT 3000 FT OVCST AND 8 NM VISIBILITY) AND IT TOOK ME ABOUT 30 SECONDS TO NOTICE HOW QUIET IT HAD BECOME ON THE ATC FREQ; IE; IT WAS TOTALLY SILENT! I LOOKED DOWN AT COM #1 AND WE WERE ON FREQ 120.70 -- NOT CORRECT! WE WERE 20 MI OUTSIDE REKKS AND SHOULD HAVE STILL BEEN ON THE TRACON FREQ. IT WAS GONE FOREVER (NEITHER THE FO NOR I HAD WRITTEN IT DOWN ANYWHERE); AND I TOLD HIM TO TUNE IN ORD TWR ON 120.75. HE DID SO; BUT COULDN'T CONTACT THEM DUE TO FREQ CONGESTION. IN THE MEANTIME; I SAW TFC (ON TCASII) JOIN THE FINAL 5 NM AHEAD OF US. I EXTENDED THE LNDG GEAR; SLOWED TO 170 KIAS AS QUICKLY AS I COULD TO STAY BEHIND HIM. THEN OUR DISPATCH CALLED ON THE ACARS UNIT AND SAID TO CONTACT TRACON O 128.45. WE DID -- THE CTLR INFORMED US SHE'D HAD TO BREAK TFC OUT OF THE APCH BECAUSE OF OUR MISTAKE. THINGS I LEARNED: WRITE DOWN ALL FREQ CHANGES! BOTH PLTS VERIFY ALL COM CHANGES ON THE RADIO IMMEDIATELY -- DON'T WAIT TO NOTICE HOW 'QUIET' IT HAS BECOME! CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THE FO HAD PRETUNED THE SECOND COM TUNING HEAD TO THE TWR FREQ. WHEN HE THEN TUNED THE FIRST HEAD TO THE WRONG FREQ; THE CREW DID NOT HAVE ACCESS TO THE OLD FREQ.