DeltaAirlines is right abt the planes - however, that was ALL the B757s that SQ ever operated. For the regord, their former regn was 9V-SGL (now N750AT), 9V-SGK (N751AT), 9V-SGN (N752AT) and 9V-SGM (N757AT).
And as for Zizou's comments, well, SQ's actually DID operate some B737s (in the early-mid 70s I think) - they inherited some B737-100s and -200s when SIA was formed with the split of MSA (Malaysia-Singapore Airlines) in 1972. But, they've only used is for a couple of years before disposing them and "standardising" on B747s and B707s then, with DC-10s and B727s to follow later.
I've got the above information from Bill Harms excellent airliner census web site - check it out at http://www.bird.ch/bharms/asr_sh00.htm
Look under Boeing 757s and Boeing 737-100/200 - you could see some ex-SQ aircraft (FYI, SQ's Boeing cust. id/number or whatever you called it was 12, so any Boeing aircraft with -x12 was ordered/made direct for SQ. e.g. B747-412