Audio/gear changes a lot faster than it used too. Since the days of Dolby Pro Logic there have been extraordinary advances in surround sound. Ever since DVD and the use of digital surround formats Dolby Digital and DTS there has been a steady developement in surround sound for home theater. These formats used SPDIF (signified by optical or coaxial cables) to transmit digital surround to a capable surround sound receiver.
On a completely different front, the video side, came the common use of Component video when DVD hit big. It uses 3 RCA connectors to handle video exclusively. It is pretty much standard that every DVD player short of some of the very first limited function and generic DVD players could use component video for an enhanced picture.
Then there was HDTV. HDTV is digital as well as DVD and is converted to analog for output via component to an HD monitor or HDTV.
HDMI combines sound and video transmission into one cable. It is also purely digital until it reaches where date streams can be processed from it either in a TV or surround sound receiver. The bandwidth of the HDMI allows for very high bitrates. So super nice HD video and very high quality sound beyond that of SPDIF can be experianced. (it is noteworthy that the video performance of component video can match that of HDMI)
Blu-ray and HD DVD, two competing High Definition video formats fighting to take the place of regular DVD, offer High Definition video and sound. HDMI can transmit all this down a single connection. Connecting the HDMI from the HD DVD or Blu-ray player to a capable receiver allows the user to get the high quality sound from high resolution audio formats Dolby Digital Plus, Dolby Digital TrueHD, and DTS-HD (MA). Then using the HDMI output on a receiver allows you to connect to your display to get your High Definition video.
Concerning your speakers, they are power hungry, and I agree that you need some power. Might I recommend getting a receiver like the Onkyo TX-SR705 and use the preouts on the receiver (basically use it like a preamp) and get a two channel power amplifier
like this one to power the Infinitys?