I have just bought a BenQ E2220HD monitor (my first flat screen) but as usual it doesn't come with either an HDMI or DVI-D cable. My graphics card (4850) only has DVI-D ports but my motherboard also has an HDMI port. Am I right in thinking I can only connect the monitor to the GPU, and the display ports on the m/b are only for the on board graphics.
Correct. You would want the DVI cable connected between your monitors DVI in and one of your graphics card's DVI outs.