I use Audacity all the time for voice-acting. That's one of the best free ones that I know of. Unless you wanna drop the cash for Adobe Audition. A cheaper solution would be Goldwave, which I've used for voice-acting.
Though, with the right plug-ins and some futzing with settings, you should be able to do quite a bit with Audacity. There are numerous tutorials out there for voice-editing, such as a Dalek voice tutorial using the Killer Ring ring modulator plug-in. I think there was a stormtrooper com-unit tutorial for Audacity, too.
Even though there are programs out there that're helpful for voice-changing, they can only go so far if the voice actor doesn't have a wide range of voices already. If you can do a variety of voices, then you should be fine with Audacity. If not, then get some friends to record lines for you. A program can only do so much.