Use on-board sound or go discrete?

Hey guy's I'm wondering what should I do.

I've got a P8Z77-V Pro Mboard , my old Yamaha 400 series (non hdmi) 500W 5.1 AMP and some Energy RC-Micro Speakers (grabbed them on sale never used review)

Now this by all accounts should be a quality sound setup for my PC almost exclusively used for gaming and music for my study.

With most recent PC titles supporting 5.1 audio I am wondering how I should set this up.

Can I just use the on board spdif digital out and with the DTS Connect + Neo and expect 5.1 signal perfectly sent to my amp to be decoded internally and amplified out to the speakers or should I be getting a better sound card to do the same thing or use the analogue output of a quality sound card and the analogue 5.1 input on the amp and not use the decoder at all?

At the moment I'm using an old SB PCI-E X-Fi Xtreme Audio and some Logitech Z2300's so I've not done a multi channel setup before on the PC or actually used the on board audio.

Thanks in advance.

With a machine being used for decent music and gaming, I would recomend a discrete sound card or use the digital output, as the onboard soundchip is usually the cheapest they can find and can useup quality ram while playing games....