If you are getting sattelites you may want to keep your sub. It can help with the lower frequencies the smaller speakers cannot produce- unless you intend to get a better one. Used electronics don't go for very much these days. I basically gave away most of mine when I upgraded. A friend at work got my old receiver and four speakers plus sub for $50. I was happy at that point just to get rid of them.
You could do things in stages- keep the speakers and upgrade just the receiver to a 5.1 (these days I would get one with HDMI). Or is it a size issue? I used to have a nice sat/ sub set from Cambridge Soundworks. I still use a pair of the speakers in my room.
http://www.cambridgesoundworks.com/s...ry=hometheater