Lena, I too run matched speakers all around. Wharfedale Pacific Pi40's up front, Pi20's in the rear, a Pacific Center, and a PowerCube sub. I like having the matched drivers because it lends to more seamless surround between zones.
IMHO, if one is going to upgrade a piece at a time, take your time and save-up for the highest quality that you can afford. It will serve you better in the long run. If you go out and buy lower quality stuff to fill gaps, you'll probably end up tossing all of it at a later date.
Till next time,
Mix