$600-$900 is a good amount of money for just an amp and speakers. You could probably squeeze a head unit into that budget too if u got ur stuff online at a site like ikesound.com, especially if you're able to install it yourself. A pair of Infinity kappa 6.5" coaxials will run you about 85 bucks w/ shipping, another $130 for 3 way 6 x 9's in the back. With about 500 bucks left, u could definitely get a decent head unit and 4 channel amp off ebay. Watch out if you aren't able to install it yourself or with a friend, cuz professional installation is gonna be pricey. I just threw the infinity's out there as a suggestion because I've heard them and the sound is very clear, not as strong on the bass though IMO. Givin up the 6 disc was hard for me but i'm used to it now because I listen to so many different CD's all the time it doesn't bother me as much. Anyway, with your budget you could definitely just get great speakers and an amp and the head unit won't be holding you back THAT much. If you do that be sure to get an amp with speaker level inputs and be prepared for a joy if u plan on wiring it yourself. If you can, I'd give up the 6 disc for a cooler mp3 head unit where u could put tons of mp3s on a cd and listen to that all the time.