I can't speak for the HP laptop, can I can relate my experiences with my Toshiba Qosmio laptop, which has a very similar configuration to the one you describe.
My Qosmio has HD DVD playback capability and HDMI output, which I connect ot my Hitachi 1080i HDTV with a HDMI-DVI conversion cable. Playback I would give a 9 out of 10. Picture quality is perfect... Sharp, well defined, definitely a step up from standard DVD. To my eye, I can tell a noticable difference. I give it a 9 due to the fact that for about 5 seconds every 10 minutes or so during playback, I see the video stutter very slightly. My wife swears she doesn't see what I'm talking about, but I swear it's there. Testing continues..
Video is output in full 1080i (for my set), and would be 1080p for a compatible display.
I love this machine. If you're not gung-ho for HP, I would recommend it. I have the Qosmio F45-AV425. However, if the HP has comparable performance, you won't regret the purchase.