Pretty much any modern PSU should be fairly quiet when at a low HTPC kind of load, though some are definitely better than others. There aren't that many lower-wattage modular units anyway. Check SPCR:
Best would be like Seasonic X Series, Enermax Modu87+ (Lepa G500), etc. for something quiet and modular.
If those are too expensive, the Antec Basiq Plus 550W is modular and is going for a very nice $45 now:
Not that much is known about models in the Basiq series other than that they're clearly Antec's lowest-end range. The Basiq Plus is by Delta though. It shouldn't be bad.