I have had a pair of radio shack Realistic NOVA-18 speakers for several years, and they were only used as the L/R channel in my surround sound DVD system. (Only used for watching movies). When the woofer surrounds finally bit the dust, I had a friend rebuild them for me.
He installed a Daytona audio DC28F silk dome tweeter (http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=275-070) and a DC250-8 10" Classic woofer. (http://www.partsexpress.com/pe/pshowdetl.cfm?&Partnumber=295-315). He also used a Daytona crossover (not sure which Hz rating), and lined the walls of the cabinet with some kind of polyfill. The woofers and tweeters are rated 89dB, and the links above even have impedance graphs to check out.
Will this be suitable for someone just beginning to tentatively step into high-end audio? At least for the time being?