How do I get 5.1 dolby digital surround sound from imac?

Hi everyone, I am very very new to this forum so if I do something inappropriate, I apologize for that. =)

Here are my questions.
I have got a mid 2007 iMac and a Creative inspire p7800 7.1 speaker.
What I want to do is to hear 5.1 dolby digital surround sound from the creative speaker (which does not support DD and DTS like Logitech z5500 and Logitech Z906 do).
I have done tons of research and spend a lots of time in the internet to look for answers. I did find some solution on this issue but I am not 100% sure. And some more questions pop up. So I am hoping that some expert here can clear my mind. Thank you so much ^^

So, here is some solution that I got from the internet. (please correct me if I am wrong)

1. Buy a Griffin Firewave and connect it to my mac and thats all I need to do. As Griffin Firewave has dolby digital processor, so I think I will be getting 5.1 DD surround sound from the speaker. (Am I right?) Griffin 4019-FIRESS FireWave Surround Sound for Macs: Electronics
But the problem is, there is only one firewave listed on amazon and one listed on ebay. The amazon one is really really expensive that I think it is really unreasonable to buy it. So as I may not able to get a Griffin Firewave, this lead to my second solution.

2. Buy a external sound card like this: Diamond External Xtreme Sound 7.1-Channel USB Audio Device: Electronics
and connect it to the mac. But my question is, As I do not have any Dolby decoding device connected, will I getting 5.1 DD surround sound in my speaker? Or the program in my mac (i.e: VLC) already done the decoding and output as 5.1 multichannel analog so I don't need a extra decoding device?

3. As I am not sure about those method on the above will get 5.1 DD surround sound in my speaker, I keep digging and find out there is actually one more option.
Since the iMac supports s/pdif output, what I need is to buy a Digital to Analog converter(DAC) and use the mini Toslink to connect my mac with the DAC. Then connect my speaker to the DAC. Some DAC I found: Digital to Analog Audio Decoder Converter - Transfer the DTS/AC-3 Digital Signal and Stereo(R/L) Audio Signal into 5.1 Analog Output: Electronics Digital to Analog Audio Decoder Converter - Transfer the DTS/AC-3 Digital Signal and Stereo(R/L) Audio Signal into 5.1/2.1 Analog Output: Electronics Sewell Direct SW-29767 Digital to Analog Audio Converter: Electronics
and most of DAC support DD/DTS decoding, I should be able to get 5.1 DD surround sound at the end. Is it correct?

My question is, is this the right solution to get 5.1 Dolby?
If Griffin Firewave is available, will the result of method 1 same as the result of method 3?
Or is it better to use a receiver instead of DAC?

I know I have so many questions, I hope I did not make it too troublesome. But I really want to know the answer from you guys. I really appreciate for those who read through this. I know my english sucks and cannot express myself very well. I do apologize for that. Anyway, please leave any thought on this, even a sentence or yes/no XD
Thank you sooooooo much