Depending on your card you can probably do both.
The DIGITAL method. With the 3.5mm (headphone size jack/socket) SPDIF output on the back of most soundcards (specifically Creative) you need a 3.5mm to coaxial adaptor lead, Creative sell one on their online shop for a few £/$'s, then get a digital (must be digital) coaxial cable to run from that to your Surround Amp (i assume you are talking about proper home theatre system, not a crappy computer 5.1 thing). This method however will only give you surround sound from a 5.1 encoded source, basically a DVD. Some games state DD5.1 on their blurb but are not truly 5.1, i havnt tested Half Life 2 yet to see, but given the size of the files its possible it is.
The ANALOGUE method. This is for when your soundcard either decodes a DVD/games 5.1 itself and just sends 6 separate signals out from its 3 rear output sockets, or when the soundcard upmixes a stereo source to surround sound (ie an mp3 or older game). Your soundcard if its 5.1 capable has 3 output sockets, one will be stereo front left/right, the next will be stereo rear left/right, the third will be centre/subwoofer (in the case of 6.1 Audigy centre/subwoofer/rear centre, i dont know about the 7.1 audigy as i only have the 6.1...lol). Into each of those 3 sockets you put a 3.5mm to stereo twin phono adaptor (must be stereo, not mono splitter twin phono) and then connect the twin phono ends directly into the appropriate External Inputs on the back of your surround amp (front left to front left, rear right to rear right etc, if you goto Creatives online knowledgbase and search you can find a picture guide that also tells you which is the centre and which is the subwoofer connection, or just try it and if you get voices from your sub then switch it around..lol)
I have both of these methods hooked up, i dont really use the digital one all that much tho as i have a kickass DVD player in my hifi setup for watching movies...lol.
Oh, if you have a good sounding home theatre system, i recommend u get an audigy card that plays DVD-Audio discs, these are not normal DVD music discs, its a different encoding system, gives you really good reproduction and if you want a personal recommendation, REM's Automatic for the People DVD-Audio sounds absolutely spanktastic, altho you will need to have it hooked up in the analogue method as you cant transfer DVD-Audio by digital coaxial cable, you need Firewire or similar speed alternative (Denon-Link for one)
(btw, 3.5mm is i think 1/8in, basically same as a small headphone plug)