I'd say use the RCA connectors, since RCA cables are usually better shielded than line-out cables. SPDIF would be even better, but you have no SPDIF-in on your speakers as far as i can see.
Unless that audio-in on the speakers is mini-SPDIF compatible, but the quality won't be as good as optical spdif anyway, since mini-SPDIF is usually coaxial
Optical SPDIF works over a Toslink cable and usually provides the best sound
SPDIF is by far the best quality. But other than that, RCA's are king. Get the gold plated connectors with the shielded cable, and you'll be fine. Remember to get high-quality cable, as (as I like to believe), half of your system's quality relies directly on your cabling. Crappy cables = crappy interference = crappy signal = crappy amplification = crappy filters needed = crappy processed sound.
Paying up to half the amount as you did for your high-quality system just for cables really isn't an uncommon thing when it comes to audiophiles.
Just for the interest of it, I saw a B&W sound system the other day (just the drivers, no amplifiers or anything included) that sold for $11,000. When I asked why they were so expensive, I was informed that the calbing (literally just two 4m cables to connect the speakers to an amplifier) cost $2000. It was good cables though, every cable had two independently shielded cables inside them, one for the lower-mid bass and the other for treble. I was impressed...
If you want to go the the "next level" you need to get digital out of the PC and convert it to analog in an external box (converter). Usually USB out of the PC. S/PDIF is more common in the audio world but is prone to some errors that degrade the sound. Then connect RCA's from that converter box to your speakers.
But that's expensive. Look up a USB DAC on audiogon under "CD-DAC". They start about $1000, used. Here's a nice one:
my sound card has an audio in / out (headphone / microphone) then on the far left there are 2 larger ones that say opt in and opt out then on the right side there are 2 that say l and r. the opt in and out would be the highest quality to connect to correct?
Two of the connections on the speaker go to the other speaker I would say go with a high quality RCA connection. gold plated . I know there are some out there that are gold plated but it will rub off if you use it about 10 times make sure you get high qulity ones.
If you need made to order monoprice.com has a lot of good stuff and is good quality.
Optical -out to optical-in over Toslink would be the best quality
That's how my FullHD media player is connected to the surround system. The sound provided over hdmi is worse IMHO.
Additionally my player can send the audio data in SPDIF RAW over optical, so DTS is not tampered with.
OP, for what your looking at a simple 3.5mm stereo jack cable. That's basically a cable that looks like it's got a male headphone jack on each end. Unless you are laying the cable next to something like a fridge any old cable found dime a dozen at Home Depot or Radio Shack will do the trick. If you don't like the generic stuff head over to Monoprice.com and pick up a heavy one for a whopping $2.50.
Just plug one end of said cable into the "front" speaker jack on the sound card and the other end into you speakers and you're set. There are no other connection methods for these speakers.
After looking at the speakers again I noticed, that the RCA connectors found on them seem to be sub-out, instead of audio-in. So the only way to connect the speakers to your pc would actually be a 3.5 mm cable (it will also be of the least possible quality compared to RCA, coaxial or optical SPDIF)