Sigmatel onboard HD ==> Logitech Z5500 speakers

Hi, new kid on the block although not to Tom's Hardware - brilliant resource for years now.

I'm hoping someone can spare a few moments to set me right on my connection options...between my onboard sound [Sigmatel HD] and my new speaker system [Logitech Z5500].





The Z5500 is supplied with a single cable of 3 X 3.5mm jacks (lime, black, orange) intended for front speakers, rear speakers and sub. The onboard hd notes suggest connecting the orange to a sub and the black to a multi-channel capable speaker setup.

There is an optical SPDIF which I am going to try anyway once I've bought one. I'm assuming that the onboard sound, if configured correctly, will output a digital stream using the optical and this will let the Z5500 do all the processing. Thus, DD5.1 and DTS from DVDs should sound great.

Now the bit I'm not so sure about...for games, can I still rely on the optical feed only? I'm presuming that the onboard HD sound card will generate surround sound but will I need to have analog connections between it and the Z5500 (in which case how do I connect them) or should it be able to convert and send the analog as a digital stream via the spdif? Ideally I want to check out the sound quality before deciding to upgrade to a separate sound card.

For those who have already trodden this path, is an upgrade probably inevitable? I've surfed quite a bit and the view seems to be that the Creative cards have slipped behind the Asus Xonar and the Auzentech X-Fi Prelude? Connection-wise, would either of these cards allow just the digital optical connection when gaming or is it required/better quality to have the analog connections?

Any help/advice much appreciated.