I've got a FAQ on HDMI in my sig that might be handy, but basically HDMI uses the same video format as DVI (since it borrowed the format
from DVI) so that you can use HDMI-to-DVI adapters or
cables to connect HDMI devices to DVI devices.
You cannot deliver HDMI audio signals to the 990. This is because when the 990 was developed, the standard at the time (version 1.1) was still very new and
two revisions were known to be in the horizon. There was no way to assure any sort of upgrade path to support those future revisions or to know how v1.1 would fit into the marketplace. The only HDMI sources that really offer anything different over HDMI audio connections than over regular coaxial or optical digital audio connections are HD-DVD and Blu-ray, as my FAQ explains.