I put Boston Acoustics S60 component speakers in this winter. There is a thread showing how I fit the tweeters in the door sails. If you can't find it, let me know and I'll search. I originally ran them off the head unit. My RS 6 came with an "aftermarket" Pioneer nav unit which had decent power. The speakers sound great. I bought them online for around $80.
Last week I installed a 4-channel amp. It is a Polk Audio PA-660. It isn't louder by a huge amount. It still rocks enough to embarass my daughter when I pick her up from middle school.
I can definitely tell a difference in the clarity with the amp. It was a good addition. I built a platform and mounted it under the driver's seat. So far, I have only wired up the Bostons through the amp. The rear speakers are still powered by the HU. I bought the amp from cowboom.com which sells open stock from BestBuy. It came with a 30 day guarantee. It only had a few small scuffs, it works perfectly. Cowboom is kind of hit-and-miss with what they have in stock though. I think it was $100.
My next purchase is going to be a sub. Cowboom HAD 10 inch Polks for around $30 and a few other brands as well. They have been sold out for a while, so I'm waiting for them to restock. I won't have time to put it in for a few weeks anyway. I plan to build a box that will fit under the passenger seat so I don't lose any cargo space.
I may bridge the rear channels on my amp to power the sub until I can afford to buy a dedicated sub amp. I was looking at the Polk PA 880 monoblock for that.
I'm certainly no audio expert. I just researched as much as I could and bought what I could afford. There are many knowledgable audiophiles here who can pipe in with