I went through this a while back with these same speakers, and here my thoughts.. ((Granted its a personal opinion but here ya go))
I had a set of x530s, and the sub-woofer was shoring out, so was looking for a replacement. They were old and abused.
The 530s weren't great, but they were not horrible either (when they were working) For the price,they were a good 5.1 setup.
At the time, I could not find any anywhere, so picked up the z506. Im sorry to say, they sounded horrible to me. They were OK for movies but music and games just sounded bad.
In the end, I exchanged them for a set of 2.1 Klipsch, and honestly happy I did. tigherter stronger bass, good mids, excellent highs.
They literally blew both the 530s and x506 out of the water in terms of sound quality.
Granted, they are only 2.1, but if you have a good sound card like a X-Fi, you can create a Virtual 3D for games. Its works Ok, but no where near as good as real 5.1 obviously. But doing that with headphones works great for games.
Just my opinion obviously, but if you are truly set on a set of 5.1 in the $100 or less range, go with the x530s if you can find them.
Ive had both, and the 530s are better (to my ears) than the 506.
But for music and strictly sound quality ... The 2.1 Kilipsch hands down.. no contest. niether of the 2 5.1s can even come close to competeing.