i have a sony 100w 5.1 reciever/satellite combo. Would it be ok, in the interests of slowly upgrading, to replace the two front speakers with tannoy mx4floorstanders? How would this affect the sound quality?
Yes you will be able to upgrade in bits alot of us do
You will be able to change the front left and right speakers to the Tannoys at a later date if you wish without too much trouble. The only issue in doing so is that the center may not match the Tannoy's tonealy and may be slightly distracting. You could buy the matching center at the same time though which would be the solution. The rears would be fine left as they are though I'd imagine. In my set up I've got Linn fronts and Mission rears and they work together well. As long as the rears are all matched and the fronts are you should be ok.
Make sure the amp can cope with the speakers you choose if they are 4 ohm then make sure the amp is able to drive them properly.
As to how they will sound you'll have to audition to see. When you feel the need to upgrade come back with a budget and you'll be able to get some suggestions for a short list of makes to audition.
Larger cabinate speakers (bookshelf/standmount and floorstanding) tend to be better than satalite speakers in every way. Offering more bass and mid range than smaller speakers gives obvious benefits when listening to movies and especially stereo music.