Depends on what you need;
As for DVD's, they should already have a 5.1 Dolby Digital/DTS track, so you can still get 5.1 without a soundcard by making sure the source audio is sent to you're reciever PRIOR to being decoded (most movie playback software on the PC tends to have software decoders built in). Its mainly for games that you need a soundcard/encoder combo to get 5.1 over a SPDIF connection.
As for the HDMI, remember, even if you can transmit 7.1, if the source is 2.0, thats all that will get sent. Make sure you are using a 7.1/5.1 source when using the HDMI connection, and try again.