Reason I asked what your running is to see if you have already upgraded front speakers, if you've got an amp to drive them and power output you have to use.
Remember that it's a system, it all works together.
In my opinion, sink your cash into a good set of front splits and a 4 channel amp, put the 6 x 9's on the back burner for now.
I'll explain why, It'll be an essay, but stick with me and learn...
The idea of a quality audio system is that if you close your eyes you should be able to picture where each "instrument" is positioned around you.
Keep this in mind while you read on...
You already have a good source of bass from your sub. Usually this would be the last thing you add, as I8AFRE has pointed out, a system can sound very good without a sub if it's set up properly.
You already have 6 x 9's in the back, which are only going to be your rear stage... You would probably be amazed how much better they sounded if you ran an amp for them.
Your front speakers should be where you invest more money. Do you go to a concert / club to enjoy the music with your back to it?
I hope not...
So why have all the sound coming from behind you in your car?
So now your saying "But subs are always in the boot, even on the best competition cars. That's coming from behind you..."
That's right.
Here's why you can get away with it. The higher the pitch of a sound, the more directional it will be.
Ever try to guess which direction thunder is coming from?
How about a whistle?
This is why splits are better for front speakers.
Instead of having all the high pitched sound firing into your legs, it's pointed at your head where you can hear it

. This makes them sound much clearer.
If you want to test what sort of difference this makes, go to a car audio store.
While listening to a set of splits move around them.
You'll look like a freak, but you'll get what I mean

.
The big drivers down lower in the door will still pump out your mids and higher bass frequencies nicely.
A good set of 6" or 6.5" splits driven by a good amp will give you a great sound experience. Your sub will take care of the deep stuff the splits can't handle. The rear speakers really just help to surround you more with sound, but they will benefit from the amp too to help balance it out.
How much you spend depends on how good you want it to sound and how loud you want to go. Better quality = better brands, how loud = more watts (Better brands will be more efficient here too though...)
The good news is that you already have a way to find a good system... your own ears

.
We're all different, and what sounds good to me might sound like crap for the next person. Listen to as many combinations as you can and decide on what YOU like the sound of before you hand over your hard earned...
Lastly a quick lesson in power ratings for amps and speakers while your shopping-
There are generally 2 figures used by manufacturers for power, RMS and PMPO.
Most people in the know don't even look at a PMPO rating, it's advertising wank in most ways.
RMS is a much more accurate way to guage the power handling of amps and speakers.
Any salesman who trys to sell you something based on PMPO values should not be trusted...
The 50W or so quoted on most decks is a PMPO value. In reality, this is closer to 10 to 13 Watts RMS in most cases. Hardly exciting stuff.
Any speakers running off this won't blow you away no matter how much you pay for them. They can sound good, but they need to be amplified to realise their full potential, as Shahrez is pointing out.
If you've made it this far you've probably got a different perspective on car audio systems. So go out window shopping and see what's around and what sounds good. Take a CD of YOUR favourite music and get em to crank it up for you. Just watch out for the guys that are only trying to pry the money from your wallet. If nothing else, you'll get to listen to some good tunes on some great systems... But if your smart, you'll find what your looking for and can budget accordingly
Enjoy.