HD out...mobo vs. vid card

Greetings,

So I've just installed Vista (not by choice actually) and I've been thinking of setting up a HTPC. My motherboard has HDMI out (Gigabyte GA-MA69GM-S2H) my video card (BFG Nvidia GeForce 8400GS) does not. I have my monitor connected to the video card, speakers out from the motherboard. Is it possible to use the HDMI out from the motherboard for both video and sound to my TV/receiver? I tried today and it seemed like my PC would not detect the TV via HDMI. I have used the other out from the video card and shown video on the TV without sound.

Any suggestions on how to get this to work? Or should I just get a new video card with HDMI out (assuming that will send sound also?).

One other quick thing...I used to use mediaportal on my xp system, but it seems to have difficulties with Vista. Anyone know of something (other than the built in media center with Vista) that is compatible?
Thanks