Z506 can be bought for £42 but used X-540 look better and perform better!
If you want 2.1 the Genius SW-G2.1 1250 are bang on you budget, look good and sound good.
Any speakers can be played in stereo using software
The left, right and centre still cost $1000 and you want better for $500 i don't understand how!? The price suggests they are still quality speakers, i would save up more money to get better!