How can I improve my computer's sound most effectively?

I currently have these speakers:

http://www.newegg.com/Product/Product.aspx?Item=N8...

Which I am fairly happy with. I am running them with my motherboard sound which is ASUS: http://www.newegg.com/Product/Product.aspx?Item=N8...

However, since there is only one input for the whole speaker system, they are only hooked up to the main channel.

I have been thinking for a while about buying new speakers. I probably want 5.1 sound (I've heard friends sets and like the immersion quality) and I like Logitech. I have been looking at the following speakers:

http://www.newegg.com/Product/Product.aspx?Item=N8...

$150 is about all I'm prepared to spend right now. However, with the motherboard sound, do you think I'd be better off upgrading to a better sound card? Which do you think will make the most audible difference? I listen to everything from punk rock to opera as well as video games, so something all around is desirable.

Thanks!