A B737-500 FLC IS OVERWHELMED BY PROCS AND WORKLOAD AND DSNDS OUT OF THEIR INITIAL APCH ALT DURING A RWY ASSIGNMENT CHANGE WHILE SETTING UP AN APCH TO RWY 17L AT DFW; TX.

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ILS RWY 17L ALTDEV AT DRAAK; 4000 FT. ATIS WAS ADVERTISING RWYS 17R; 17C; 17L. AT LAST MIN A TIGHT TURN TO FINAL FOR RWY 17L. THEY ASSIGNED IT WHEN WE WERE SET UP FOR RWY 17R; THE USUAL AND PREVIOUSLY ASSIGNED APCH. ON VECTOR FOR THE L SIDE WE QUICKLY DIALED IN RAW DATA RWY 17L. WENT TO PUT IN BOX AND WE FOUND SOME INTXNS NOT ON THE APCH. WE QUERIED ABOUT THE TURN ON AND WERE TOLD JUST OUTSIDE DRAAK AT 4000 FT. WE HAD TO PUT IT IN THE BOX AS WE TALKED; DIALED; TURNED; TYPED AND PUSHED BUTTONS. FO FAIRLY NEW SO COULDN'T REALLY BACK ME UP. SOMEWHERE IN THERE WE DSNDED THROUGH 4000 FT BEFORE DRAAK. WE MISSED A BUTTON AND DSNDED AS WE DID ALL OF THIS PLUS CONFIGURING AND COMPLYING WITH SPD ASSIGNMENTS. CAUSE: ATC IS CHANGING AND COMPRESSING SITS TOO MUCH IN ORDER TO INCREASE TFC FLOW. WITHOUT A FLYING BACKGROUND THEY ARE NOT AWARE OF WHAT NEEDS TO BE DONE OR HOW MUCH NEEDS TO BE DONE; THUS SETTING UP FOR MISTAKES. WE TRY TO COMPLY BUT PROBS OCCUR WHEN THEY ARE ASKING US TO FLY STRICT IFR COMPLIANCE APCHS AND WHEN TRYING TO COMPRESS LAST MIN TYPE CLRNCS THAT DON'T GIVE US ENOUGH TIME TO DO THE JOB. COMPUTER WORK ADDS TO THE PROBS IN THESE SITS. PREVENTION: ADVERTISE AN APCH AND STICK TO IT IN AN ORDERLY SET STABLE PROC. USE MORE CTLRS OR GET BETTER EQUIP (ATC). BOXES (FMC) NEED BETTER AND MORE STORAGE CAPABILITY (AIRLINE).