Does the impedance have to match?

I have a cheap all in one receiver/dvd. I want to upgrade the speakers that came with it. All the speakers that came with it are 3 ohms including the passive sub.
My question is do I have to change all the speakers to have the same impedance including the sub. Or could I just change the front and center speakers. It is connected to the tv in the livingroom. I only use it for watching football and movies. I have a 2ch set up in another room for music. My girlfriend likes the way it looks in the living room, so the receiver has to stay. I just want it to sound a little better without blowing it up.
No you don't have to match impedance. In fact you're better off getting higher impedance spkrs -- & most after market spkrs are higher impedance anyway. Go for very "sensitive" spkrs i.e. their "dB/1w" rating should be 90 or more.
Agreed. But, if this is a home theater receiver, put your money in the center channel. That is where all the info is coming from to begin with. The fronts barely do anything in HT setups (for the most part). I would think that this would have the biggest impact in what you are trying to achieve...especially with movies.
Yes it is a surround reciever. So you guys are saying that I could run say 8 ohm speakers for the fronts and center. Then it would be ok to leave the rears and sub with the 3 ohm speakers that came with it? I was under the impression that I would have to change them to 8 ohm speakers also. Thanks for your input.
In general, it is a good idea for speakers to 'match' in the sense of similar timbre or pitch response.
So, while technically it should be possible to run 8ohm speakers as fronts / center you may find yourself being able to point at them instead of simply disappearing as perhaps they should. Depending on your tolerance to this, you may be able to get away with a less than perfect match.
Also, Gregm's comment about sensitivity are well found. Doesn't appear you have much power here, so make the most of what you got!

Christmas is coming and 'upgrade fever' can strike at any time.
At the risk of sounding like a broken record, IMHO amplification makes a poor foundation for choosing speakers. Better to use speakers as a foundation for choosing amplification.
Good point Unsound, but this receiver is staying put for the time being. My girlfriend liked it and bought it. I put all of my money into the 2 ch system that I have listed this site. I just can't stand the sound of the crappy speakers that came with the receiver much longer. My girlfriend has agreed to let me change the speakers, but likes the all in one receiver/dvd player unit. I'm sure with some better speakers it will at least sound more tolerable. Down the road i hope to talk her into a better set up, but for now i'll have to work with this. Thanks for your suggestions.