Some switchers run at frequencies of up to 3MHz, having said that I've had problems with lower frequency switchers being very noisy.
EMI is a real issue for switching regulators. I remember building a DCRCD and using a DC-DC converter module for to get +/-15V@50mA from 18-30V or 36V to 60V (depending on the model). When they tested it for EMI, they were prety supprised to discover emissions at 76MHz which is pretty high considering the switching frequency was only 300kHz. This wasn't my fault because, for a start no one gave me a specification for EMI and the DC-DC converter was an industrial standard off the shelf part, if I had know about this then I would've used a linear regulator power rail splitter for the 36V to 60V model and a charge pump converter for the 24V to 30V model.