Welcome and don't worry, we all have to start somewhere. You are heading in the right direction by pulling away from the HTIB systems like the all in one Panasonic and moving toward a AVR and separate speakers & subwoofer.
From what I gather from all above you basically have a Yamaha AVR, a Polk subwoofer and are looking for 5 speakers to mate with the Yamaha to complete a entry level 5.1 system. First thing you do is discard the Panasonic HTIB speakers as you are right - they are crap!
Second thing to know is that AVR's are rated in a very confusing manner and your receiver is probably putting out only 45-50 watts per channel, all 5 channels driven across the full frequency range of 20hz - 20khz. This is really the only spec that matters - not the hyped up marketing bs of 85 wpc into 1 khz. Despite the actual lower wattage number, this is OK too if you get the right speakers.
So, I would suggest you got back to Best Buy and get some entry level Polk TSI series, to match the Polk sub you aleady have. The TSI speakers are fairly efficient at around 90 db and most important - they are 8 Ohm speakers. 8 ohm speakers are much easier to drive than speakers rated at lower impedences such as 6 or 4 Ohm. Stay away from these! With the small built-in amp in your Yamaha, just stick with 8 Ohm and sensitivity around 90 and you'll be fine. You will be in way to a decent entry level 5.1 system that will be light years ahead of any HTIB.
