Why 5.1 speakers?
We should be asking why you want 5.1 speakers here...
Don't expect to have full 5.1 surround sound when playing games. Certain applications (like DVD Player) do use the 6 channels very well, but others are still "working on it". OpenAL is better supported these days, but it is still a ways off.
iTunes seems to play through all 6 channels, but it is not "coded" for 5.1, so it is just "more sound" than 2.1 speakers.
I have the Revolution 5.1 card in my G5 and I love it, but I expected more out of it than I am getting. Here's to hoping the future brings OpenAL support to more games.
There is also one other things to consider: Hardware Decoding of the signal (Dolby DTS etc). I really haven't gotten a firm grip on that knowlege, so I won't comment on it fully, but only high-end speaker systems (Like the Logitech Z-5500) will do the decoding for you.
I hope this helps.
-Brett