Those speakers are a 5.1 setup, so you can easily use digital-via-coax. With Logitech gear, you'll want to use coax for surround. The optical inputs don't always work well with surround. As for your cable... the cool thing about digital is that it'll either work or it won't, especially at the low frequencies used by audio gear. Follow masshass81's advice above. You don't need to spend much money on digital cables. Monster Cable is no better than WalMart cable for digital. Especially if you only need 18 feet of cable or less.
As for digital optical vs digial coax... there's little difference physically. The coax cable will make electrical pulses to send the digital ones and zeros. The optical cable will blink the laser or led light to represent the ones and zeros. The real-world differences deal with the actual devices being used. In your case, the Logitech gear is much better suited for coax. Using optical may limit you to just 2 channel stereo. Do your research, almost every brand is different.... *sigh*
Another way is to use several cables to bring each channel of audio from the card, in its analog form, to the speakers/amp multiple analog inputs. But this would be silly and would probably result in lower quality anyway. The digital coax cable will already carry all 5.1 channels for you.