Another approach is to use a PC soundcard. Recenctly many amateur radio application software have been developed for soundcards. Those include audio spectrum analyzers, audio filters and MOD/DEMOD software for SSTV/RTTY/PSK31 etc.
I decided to use a soundcard. It can handle audio frequency only, but many of low cost DSP evaluation boards are also restricted to AF. It also have merits. Because processing is done by PC's main CPU, you don't need to learn new programming environment and floating point calculation is available unlike some low-end DSP chips . The sound chip in my PC is CS4614 by Crystal Semiconductor. Its maximum sampling rate is 48kHz (16bit) and it has full-duplex capability.
I chose Linux and g++ compiler as programming environment. The sound driver is ALSA. Recently I began writing codes also in Java, so some of my prorams run on MS-Windows , too.
Besides those programing environment, tools are needed to design digital filters and other functional elements. Many commercial and non-commercial ones may be availble, but I don't know well about them. Currently I'm using Java applets which I found on a web site to design FIR filters.