First of all, I'm sorry to have confused you about "monitors." I was talking about monitor speakers; they are called "monitors" because they are designed to sound best when they are close to your position (such as on a desk), just like video monitors are intended to be seen up close. They are sometimes labeled "studio" because they are often used for mixing audio.
Your STX has five external connections. Front L and Front R are RCA analog connections; they are for hooking up a left and right speaker for 2.0 audio. They can also be passed through a subwoofer for a 2.1 system. There's a 1/4 headphone jack (analog), and a 1/4 input jack often used for a microphone. Finally, there's a S/PDIF out (Sony/Philips Digital Interface), which has an RCA type connection that uses coaxial cable.
Your analog outputs will use the very good components in the STX that convert digital music to analog (all music must become analog at some point - sound is analog). Your digital output will use similar components in a receiver, or sometimes the subwoofer of a system like the Logitech Z-5500. Unless you use a high end receiver, those components will be of much better quality in the STX.
The STX is a rather niche sound card for audiophiles using high end headphones or speaker pair. If you're really looking for a 7.1 gaming sound card, you really choose poorly, I'm afraid. You should return if at all possible.
I would recommend a Creative X-Fi or ASUS D Series card.
edit:
Both analog and digital outputs will use the cards processor and potentially take some load off your CPU. However, with today's CPU's the effect is supposedly small. Creative's cards and ones based off their audio processors like the Auzentech's are really much better at boosting performance than ASUS cards.