Who'd Play Who?
Yajirobe- Jeremy Piven (PCU, Family Man)
Piven would work as Yajirobe because he's a little bigger and could play a lazy guy like Yajirobe.
Hercule/Mr. Satan- The Rock (yeah, the wrestling guy...yuck)
This guy IS Hercule/Mr. Satan.  End of discussion.
Dr. Gero-Tim Curry (Rocky Horror Picture Show, Muppet Treasure Island Congo)
Tim Curry is the greatest vilian ever.  He has already proven that he plays a hilarious mad scientist and now that he's old, he's perfect.
Krillin- Seth Green (Can't Hardly Wait, Austin Powers)
Krillin is a really hard guy to cast.  Green would be acceptable because he's little and he plays a good loser.
Android 17- Stephen Dorff (Blade)
Dorff has the right attitude for 17, and he looks alright for the part.
Android 18- Angelina Jolie (Hackers, Gone in 60 Seconds, Pushing Tin)
Jolie is one of the few women tough enough to play 18 and she has the right vocal quality too.
Trunks- Johnny Depp (Ed Wood, Edward Scissorhands, Sleepy Hollow)
NO, I do not think it should be Leonardo DiCaprio, so don't even think about that.  If you think it should be him, you're a moron.
Frieza- Joan Jett (The Runaways, Joan Jett and the Blackhearts)
Who says Friza is a guy?  Sure didn't seem like it to me.  Joan Jett sounds just like him...her...it?
As of right now, I have no Goku, there's nobody I can think of to do him justice.