upgrading factory speakers

2004 Chevrolet Epica (same as Suzuki Verona in US)

My car has four speakers: one in each door, with locations on the dash for tweeters (not installed).  I want to upgrade my speakers to get more decent overall sound but without getting too carried away and preferably sticking with the stock locations.  I'd like to keep my stock headunit if at all possible.  I have a few questions which I hope will be easy for those who are experienced :)

A friend sold me  two 6.5" speakers that will fit my front doors.   (JVC CS-VS22)  They are two-way 150 watt peak.  I'm interested in putting possibly three way or four ways in the back doors (5.5").  Does this make good sense or are they usually reserved for front locations?  I'm not installing a seperate subwoofer, so would installing tweeters in the dash be too rich in the highs and sound out of balance?

Because I'm keeping the stock head unit I'm assuming I'll need an amp to power these four aftermarket speakers.   The HU doesn't have RCA on the back.  Someone mentioned to me a "Line out converter".  Is this the best way to go about this?

...and one more :)  Can I keep the factory wiring going to the speakers or is it a must that this is upgraded?

I want to keep simple but if I'm going to be doing anything I want to do it right.  Thanks for any help you can give!

I suggest going with a component set for the front end and use coaxials for the rears. Use a 4 channel amp that has at least the RMS of the speakers RMS rating and find the appropriate amp that matches this number. If it's not too high & your on a budget, you can always use a mini amp ( like the Alpine 4 channel KTP-445 ). This plugs inline of a OEM or aftermarket radio without having to use a LOC.

That looks like a great simple solution!  I didn't even know something like that existed.   Are there options out there for factory head units?  That one seems to indicate it has to be an Alpine...

Other then the max volume are there any other consequences of under powering the speakers?  If I had a 60 watt RMS rated speaker and I'm only giving it the 45 from that mini-amp will that decrease the quality of the sound greatly?

All you have to do is cut off the Alpine connector and wire in the OEM connector in it's place. What you can do is buy a reverse harness that plugs into the OEM deck and wire it directly to the Alpine harness, this way you don't have to cut the OEM plug and also the aftermarket reverse harness have all the proper wire colors on there as well.

The speakers will clip if you under power then, but not as bad with the Alpine amp in comparison to trying to use a deck to power the same speakers.

Sounds like exactly what I'm looking for.  I all ready have the coaxial speakers, and they only fit the front so I'll go with those.  I'm not sure of the RMS but if it clips, I'll make up for the low in the rear and tweeters in the dash will cover my highs.  Now that I know I have 45 watts to work with I'll buy speakers for the rear within that spec so they'll preform the way they were designed.

This solution saves a lot of time and money and I think I'll be happy with the results.  It won't be anything jaw dropping but that wasn't what I was going for anyway (as tempting as it is to get carried away)