bang for your buck does not give us a budget.
for instance there are speakers for under $50 which are good deals for that money, speakers under $200 which are good deals for the money, etc.
z313 for its extremely low price is not bad. its not excellent but will get you through a bind. the more expensive 2.1 systems are somewhat better.
creative has some good choices as well. the a250 is good value also.
while the x-series was good, you may not want to try and find/buy used.
if you have a higher budget, the soundsticks III and creative creature III are decent pc speakers. the higher end creative/logitech models are decent value.