I haven't seen a motherboard in a while that doesn't have 5.1 or even 7.1 analog outs. My P67 has them.
Also, it would be rare that your TV outputs anything other than Dolby Digital recieved from your cable/dish/OTA source. Not many have passthrough capabilities, even on very high end sets. The "5.1 Dolby Digital Surround!" on the TV box just means that it has a decoder for those sources, not passthrough, or decoding of external sources.
It's kind of a selling tactic and a shady one at that.
You would most likely only get stereo from your TV through optical from any other source. In other words, running your audio through the HDMI cable from your PC to your TV and then from your TV to your reciever using the optical cable, you aren't likely to get anything higher than 2.0 (stereo)
The analog output option on your motherboard would be fine, just get one cable mentioned above for each pair of speakers you plan to use (one for the front L/R, one for the center/sub, one for the surround L/R). *Note that this is only if your reciever has 5.1/7.1 discrete inputs! If not, you're limited to stereo no matter what you do.*
This is a pretty standard blurb concerning optical outputs on TVs -
"The only 5.1 output through the optical out on the television would come from the tuner (assuming they are broadcasting a 5.1 audio feed). HDMI surround to the television will only output 2 channel audio via optical."