A B757 FLT CREW FAIL TO CATCH ERROR IN THE FMS RESULTING IN BEING OFF COURSE AND GETTING A TCASII TA.

Narrative

I CALLED GND FOR TAXI INSTRUCTIONS. THEY ADVISED NEW DEP FREQ; TAXI TO RWY 7L. AS WE TAXIED; I REPROGRAMMED THE COMPUTER FOR RWY 7L. IT HAD ALREADY BEEN SET UP AT THE GATE FOR RWY 25R VERIFIED BY BOTH LEGS AND PLAN PAGE. AS I REPROGRAMMED FOR RWY 7L; I VISUALLY CHKED THE MAP. IT LOOKED GOOD AND EXECUTED IT. THEN THE CAPT HAD GIVEN THE NORMAL TKOF BRIEFING (INITIAL ALT; HDG; ENG OUT AND SPECIAL PROCS). WE DEPARTED WITH NORMAL CALLS AND AT 400 FT I ASKED FOR LNAV; FOLLOWING THE PINK AND CLBING TO 7000 FT. AT APPROX 6100 FT; WE GOT A TCASII TA/RA. I CONTINUED TO FLY AND THEN ATC ASKED IF OUR RNAV HAD BROUGHT US OFF COURSE. I QUICKLY WENT TO RAW DATA TO CHK AND WE WERE L OF COURSE. I THEN TURNED TO A 090 DEG HDG. ATC THEN ASKED US TO TAKE UP A 110 DEG HDG. WHILE THIS OCCURRED; 7000 FT CAME AND WENT. I INSTANTANEOUSLY BROUGHT THE PLANE BACK TO ASSIGNED ATL AND TURNED THE AUTOPLT ON. AT THIS POINT; ATC THEN HAD GIVEN US CLRNC TO NEXT FIX (TRALR) AND ON COURSE; ALSO SWITCHING US TO ZLA. BEFORE SWITCHING; THE CAPT SAID; 'SORRY ABOUT THE ERROR.' ATC RESPONDED 'THANKS; NO PROB.' I; BEING COMPLACENT; SHOULD HAVE CHKED THE LEGS AND PLAN PAGE PRIOR TO USING THE LNAV! SUPPLEMENTAL INFO FROM ACN 632728: WHILE CLBING OUT; WE FIRST NOTICED A TCASII TFC ALERT (NOT AN RA;) AND BECAME INVOLVED WITH LOCATING THE CLOSE PROX TFC. MY SHORT TAKE ON WHAT HAPPENED: MY BEST GUESS IS THAT WHEN THE RWY CHANGE WAS MADE DURING TAXI OUT; THE FMC DROPPED THE DEP PROC AND GAVE US A COURSE TO OUR FIRST ENRTE FIX. THIS NEW COURSE TOOK US TO THE L OF THE RNAV DEP PROC COURSE.