The Logitech system is very nice for the price, but beware that after a few years it failed on my friend and now is unusable.
I would go pick up a cheap stereo receiver (The Denon DRA-25 is a personal favorite) off eBay and then buy a pair of nicer bookshelves.
These are favorites of people in the industry, and they look like they are on sale.
After a while you can buy another pair and run them dual-stereo (on a DRA-25 at least) and then save up for a nicer, surround receiver.
This could work out nicely, and I guarantee a stereo setup would be better than the z5500.