Some pointers:
If you need a memory cache, have a look at memcached - a distributed memory object caching system Have a look at http://dev.robotcoop.com/Libraries/ for plugins and libs.
Rails also has great caching features built-in:
Rails includes support for the script.aculo.us autocompleter-- basically, it's two lines of code and you're there. Of course you can customize, too. Demos and example code: http://demo.script.aculo.us/ajax/autocompleter http://demo.script.aculo.us/ajax/autocompleter_customized
Best, Thomas
