Advice on upgrading speakers with Onkyo HT-R380

[NoirFan]

Hi, apologies if there is already a thread devoted to the subject, but I didn't see anything at first glance. I am looking to upgrade the speakers included with my Onkyo HT-R380 5.1 system. I'm not looking to spend more than $300 -$400 for the group, but, as I know zero about audio/speakers, had no idea where to begin. Any ideas?

Edit: I need to stay small for these, like this:

http://www.amazon.com/Polk-Audio-5-C...197949&sr=1-41