I play 720p and 1080p videos on a first-generation Core Duo MacBook (with the GMA 950) so you'll definitely be able to play such videos on a MacBook Pro with the 8600M GT. And no, you won't lose any resolution hooking it up to an HDTV (if you use an HDMI-DVI converter cable).
1080p does drop frames occasionally, though, but that's still using a far slower, older video engine. The 8600M blows the GMA 950 out of the water.