Tom Kenny was also almost every unnamed male character on the show. At least half of them. Including an actor who played an Ewok in Return of the Jedi who sounds exactly like SpongeBob. Tom Kenny was to this show what Billy West is to...a lot of animated shows of the 1990s, 2000s, and now (read: he voices a lot of characters).
Gwen is Jane Weidlin of The Go-Go's (which explains why Gwen's favorite band is The Go Gos)
Posey is Vicki Lewis. Her voice for Posey was a case of Throw It In, as she came into the recordings from the dentist's sounding really weird from being under anesthesia. Fortunately, she was able to recreate it for future episodes.
Keep Circulating the Tapes: Even though the show is out on DVD, all the licensed music has been overdubbed with generic music, so if you have any episodes from when it aired on The WB!, [adult swim], or TBS, then keep those (whether you taped from a VCR, a recordable DVD player, have them DVR'd, or you downloaded them from a torrent site). YouTube has the [adult swim] cut of the series with the licensed music, so catch them before they're pulled for copyright violations.
Screwed by the Network: The show was cancelled after only six episodes aired because the ratings weren't instantly phenomenal. It was picked up by [adult swim], but, unfortunately, Bill Oakley and Josh Weinstein had since moved onto other things and weren't interested in finishing the incomplete episodes or making new ones, so, unless Oakley and Weinstein change their minds, there's no chance of the show ever being revived.
What Could Have Been: There were 18 planned episodes, with two of them ("Freaky Weekend" and "Pretty In Pink") having partial animatics and voice recording finished, and the other three ("Death Of A Yale Man", "How To Get Head In Advertising" and "I Was A Teenage Pornstar") fully scripted. The series was cancelled before any of them could be finished. The remaining material can be found in various places online. If the show hadn't been canceled, viewers would have seen Kevin finally standing up to his parents ("Bye, Bye Nerdie" or "I Was a Teenaged Porn Star") and Ron's Waterbed Store (which was seized by the government after Ron got busted on tax evasion) would have been taken over by a man who looks like Ron, but is Southern ("Crap Gets in Your Eyes" or "Pretty in Pink").