DVI-i is a combination of DVI-a and DVI-d signals on one connector. DVI-a is analog, RGBHV just like VGA... A DVI-i to VGA adaptor picks up the analog signals with a DVI-a plug & rewires them to a VGA connector. DVI-d is the digital part, used by flat panel monitors & TVs.
A significant difference between using ATI and NVidia products for HTPC is that only ATI produces interlaced resolutions. That is not as limiting as it may seem, because progressive resolutions are far preferable for PC displays. A 1080i HDTV can be run in 960 x 540p mode, for example, for a progressive scan desktop.
The owner of the BEAUTIFUL new HLP5685W should know a couple things about using these newest Sammys... First, since you have the HD2+ 720p chipset you should use use a 720p input signal (native res). Second, these newest models from Sammy have a new way of adjusting overscan for PC use that only has about a 2% overscan AND is 1:1 pixel mapped. Check your manual for details on using the PC input setup menu. And congrats on a great set!