Han,
Best to you on your ambitious undertaking!
Here is something you may want to consider. Although the Logitech speakers may suffice for your Keyboard in a small intimate setting, you will in my opinion need an extra Uumph! not only for your Keyboard but for the rest of your Band to give you the sound you require.
Since the Behringer UB1202 mixer is NOT powered you should consider getting a PRE-Amplifier for your Mixer to boost the signal like the QSC RMX 850 Power Amplifier (used as an example only) which are sold at Guitar Centers. Then get some good Passive speakers such as the Mackie C300 Passive Speakers. They're a bit pricy but should be considered a great investment for excellent sound that will last you for years to come.
Or then again, you can forego the PRE-'Amplifier' and get some good Active or Powered Speakers. There are many out there that you can get that won't break the bank and are excellent speakers.
But IMO you need some quality speakers (whether powered or passive) that will do your Keyboard justice and also the rest of your Group too.
Just remember if you get passive speakers you will also need an Amplifier as a go between for your Mixer and speakers to boost the signal to sufficient levels.
Best regards,
Mike
[This message has been edited by keybplayer (edited 09-26-2004).]