B737 ARR FAILS TO COMPLY WITH ATC ASSIGNED ALT RESULTING IN LOSS OF SEPARATION AT IND.

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AT XA06; I ISSUED ACFT X A 340 DEG HDG AND 5000 FT; A VECTOR FOR RWY 23R AT INDIANAPOLIS. THE PLT READ BACK 340 DEG HDG AND 5000 FT. CHEROKEE WAS 15 MI SE OF IND AND WAS GIVEN A 270 DEG HDG AND WAS ALREADY LEVEL AT 4000 FT AT THE TIME OF THE VECTOR. THIS WAS FOR RWY 23L AT INDIANAPOLIS. THE SECTOR GOT BUSY AND I ISSUED THE CHEROKEE 3000 FT; WHICH HE READ BACK. WHEN I LOOKED AT ACFT X'S TAG; HE WAS ALREADY AT 4000 FT HDG TOWARD THE CHEROKEE. I ISSUED A 020 DEG HDG TO ACFT X AND TO MAINTAIN 4000 FT. I THEN ISSUED AN EXPEDITED DSCNT TO CHEROKEE AND GAVE ACFT X A TURN TO 050 DEGS. UNFORTUNATELY; SEPARATION WAS LOST 800 FT; 2.2 MI; APPROX 10 MI SE OF INDIANAPOLIS. AS TFC BUILT; MORE SECTORS SHOULD HAVE BEEN SPLIT OFF SINCE 4 SECTORS WERE COMBINED TO MY POS. THIS MIGHT HAVE HELPED IN NOTICING ACFT X BUSTING THROUGH HIS 5000 FT ASSIGNED ALT WHICH HE ACKNOWLEDGED; AND HAD READ BACK. ALSO; MOUNT COMFORT; INDIANA; HAD AN AIR SHOW PLANNED FOR THE DAY SO I BELIEVE; ALTHOUGH TFC WAS LIGHT BEFORE THE ABOVE INCIDENT OCCURRED; IT GOT BUSY FAST AND ANOTHER SECTOR SHOULD HAVE BEEN OPEN ALL DAY; NOT WHEN TFC DICTATED IT TO BE OPEN.