5.1 Surround Sound on iMac?

I own a 24" iMac for just over a year now. I was recently extremely disappointed to find out that the Creative A500 5.1 Surround Sound system does not work on my iMac! I can only get 2 speakers and sub to work, which sounds really awful when watching a movie.

The problem is that those stupid iMac sound cards only support 2-channels. And since I can't install a new sound card into my iMac, my question is, if I get an analog sound source, or a USB splitter into left and right audio, will I get true surround sound?

Very disappointed with my iMac

Those iMac sound cards most certainly do support 5.1 if you're listening to a 5.1 source and using an optical cable for your output. You will not get 5.1 if you're connecting an analog stereo mini-plug up to it.

You will need a mini-toslink to toslink adapter or cable for 5.1 output similar to one of these.

The audio port on your iMac is a dual port and supports both the stereo mini-plug and mini-toslink.

If those speakers do not have the capacity for digital input, then you'll need an external sound card.

Edit: I would personally recommend upgrading your speakers to some that do allow for optical input rather than going the external sound card route. Look for some that have Dolby Pro Logic if you're wanting to get 5.1 sound from stereo sources, such as probably all of your music.