Performance increasing question, pleas help

Hi all, I have a question. I've bought the parts and new system is on it's way (yay!!) this is my first build using a video card with hdmi... the motherboard I ordered is a BIOSTAR TA785GE 128M AM3/AM2+/AM2 AMD 785G Micro ATX AMD Motherboard I am going to use an actual video card SAPPHIRE 100256HDMI Radeon HD 4670 1GB 128-bit GDDR3 PCI Express 2.0 x16 HDCP Ready CrossFire Supported Video Card using the hdmi port to connect to a 46" hd 1080p tv with home theatre surround sound. the video card claims to have 7.1 audio. I've never used this type of setup before and I'm wondering if the sound coming from the video card is of decent quality? If it is, can I disable the onboard video AND sound (as well as the onboard nic as I will be using wifi) and by disabling those items through bios will I get any noticeable speed increase? If it matters I will be running 4 gigs of ddr2 1066 ram on vista 64.

Thanks in advance, I wanted to give as much hardware info as possible