The Z5500 is an excellent setup if you replace the stock Logitech speakers with good ones. I posted somewhere earlier in this thread the model of the Infinity's I use with the PC I'm at the moment. I've used this rig with Asus Xonar D2X and Essence STX audio cards along with Sound Forge Pro for doing audio editing and mastering work for two record companies that send me work. I have another complete Z5500 in storage as well.
I also have professional JBL and Electro-Voice systems as well as some Fosi stuff I haven't even unpackaged yet. Most of my pro gear is in storage after the forced move from a large house into a Senior living apartment unit that's about 1/5th the size of the house.