This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
A VIDC enhancer is basically a clock change for your VIDC. Most Arcs (bar the A540 and newer machines) have 24 MHz VIDC chips installed in them. A VIDC enhancer increases this to 36 MHz allowing much higher resolution screen modes to be displayed on your Arc. (800x600x16 or SVGA standard becomes available.) You do not need one to use a Multisync monitor - the standard VIDC handles that just fine. However having a VIDC enhancer is only really useful if you do have a Multi-sync monitor.
A 'build-it-yourself' VIDC enhancer is available from http://www.mirror.ac.uk/collections/hensa-micros/local/riscos/drivers/autovidc.arc and Maplin Electonics supply 9-15pin converters for around £6.00
Note a VIDC enhancer is unnecessary and incompatible with the RiscPC range of machines.