adding extra tweeters isn't the answer. It's kind of spendy, but Get a nice set of front speakers and amplify them. I really wouldn't worry about the rear, they really only serve to throw off your imaging. Ideally you should make a good quality cd that covers you favorite styles of music. And perhaps some other stuff that has alot of clarity that you might listen to as well. I really like using Dave Matthews Crash because there's alot of seperation and though it gets busy, it's all very clean and crisp if heared on good speakers. Then go and listen to a many speakers as you can. I'd look at JL, Boston, Eclipse, MBQuart, CDT, etc. Component 6.5's will fit, and actually Boston inclides 5x7 adapter plates with their Pro's this year (guess what I'm getting) WIth components, Expect to spend at least $200 for speakers, and if you're serious, I'd shoot for around $400 or so. but listen and see what you think. More money is not necessarily better. I personally was sold on some $800 focals until I heared the Boston pro's which are $450. If you want to stay simple and keep to coaxiles, you're not going to be making a huge sacrifice, Actually ever since I upped the juice on my amp, my Kappa's have been sounding pretty damn good again. As for amps, Alpine, eclipse and JL make some nice clean amps. Eclipse especially has some lower priced stuff that sounds incredible. Look to send atleast 50w per channel. and if you don't mind spending some money to sound good, you could push upwards of 150w (more power=good)
