Your HDMI port on the video card should work plug and play without having to install drivers. Install them regardless. Like Seth said, its one or the other. Your MBs graphics won't work if you have a vid card installed. I just pieced together a simple box this week and HDMI worked from fist boot prior to OS install from the vid card into UFEI.
1. What video card do you have?
2. What MB?
3. Does the monitor say no signal?
4. Check the cable in another device
5. Does the card work with other output options?
That's a good start to figuring it out, it ought to work so its probably something simple.