How big is your audience?
will make you think->
How many speakers do you need?
personal opinion or open air gig, will make you wonder->
Do I want a subwoofer?
There are active/passive speakers/subwoofers. The difference is that the amplifier is built-in the active speaker. In the passive speaker you need an amplifier "sold separately".
WIth an amplifier, just make sure it has an RCA input for connecting you usb audio card.
You'd better talk to a local friendly dealer. They are there to suggest what you need. The only advice I can give is: if you feel uncomfortable on one store, GET OUT OF THERE!
You might by interested in the U-Mix Control Pro! A fine DJ soluction from MixVIbes!

Regards,
Daniel SG