B737 LANDS RWY 1L APPROX 100 FT BEHIND B747 DEPARTING RWY 28R.

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THE WIND AT LAS WAS RPTED AS 330 DEGS AT 20 KTS; GUSTING TO 26 KTS. TSTMS TO E AND SE OF FIELD. WE WERE CLRED FOR VISUAL APCH TO RWY 1L; AND A CPR GULFSTREAM WAS SEQUENCED AHEAD OF US (APPROX 4 MI AHEAD). WE WERE TOLD TO SLOW TO 150 KTS. TWR INFORMED US A HVY B747 WAS ON TKOF ROLL ON RWY 25R. IT WAS DUSK; AND BY THE TIME WE SPOTTED THE B747; HE WAS JUST ABOUT TO LIFT OFF I WOULD ESTIMATE NEAR INTXN A7. WE WERE GOING TO LAND BEHIND HIM; BUT THE DISTANCES WERE SO CLOSE THAT I THOUGHT A GAR WOULD BE TOO CLOSE WITH RESPECT TO HIS ACFT AND HIS WAKE TURB. WE HAD AN APCH SPD OF VREF +16 KTS FOR WIND; AND I SUGGESTED TO THE FO FLYING TO INCREASE TO +20 KTS AND BE READY FOR HIS TURB. (WE WERE NOW AT APPROX 200 FT.) AT 50-100 FT; WE HIT THE WAKE OF THE B747; AND OUR ACFT ROLLED APPROX 20 DEGS. THE FO ADDED PWR; CORRECTED THE ROLL; AND LANDED. I THINK THIS SIT WAS A POOR DECISION ON THE TWR'S PART. A B747 PROBABLY NEAR MAX GROSS; ROTATING AND CLBING SLOWLY OVER OUR INTENDED LNDG SPOT; IS AN ACCIDENT WAITING TO HAPPEN. WE PASSED THROUGH HIS TURB ABOUT 10 SECONDS BEHIND HIM. A LOOK AT THE ARPT CHART WILL SHOW HOW CLOSE THOSE RWYS ARE.