I think you will learn more if you clarify your question by stating why you want a HTPC, (for example: rather than a workstation or a stand alone blu-ray player). Here is a list of the range of possible purposes for a HTPC that I collected last year, (some of this is outdated):
What is an HTPC?
HTPC stands for Home Theater Personal Computer. It is understood to be a synonym with "Media Center PC", however the HTPC name implies use with a full home theater setup. Really, the best definition for an HTPC is any computer with special harware and/or software to make it more suited to audio and video playback and/or use with audio and video equipment than a normal computer. I think the best explaination for what an HTPC is is what it is used for.
What can an HTPC be used for?
-Playing/Ripping/Burning/Cataloging DVDs
-Playing/Ripping/Burning/Cataloging CDs
-Playing/Ripping/Burning/Cataloging Blu-ray disks
-Playing/Ripping/Cataloging HD-DVDs
-Playing/Ripping/Burning DVD-As
-Watching, timeshifting, and recording analog TV (OTA and analog cable)
-Watching, timeshifting, and recording digital TV (ATSC, DVB-S, DVB-T)
-Watching, timeshifting, and recording unencrypted QAM (unencrypted digital cable)
-Listening to and recording FM radio
-Upscaling video to the native resolution of a display and processing in realtime
-Equalizing audio
-Playing/Storing/Remotely Acessing/Transcoding MP3s, OGGs, WAVs, and many more audio formats
-Playing/Storing/Remotely Acessing/Transcoding WMVs, MPGs, MOVs, and many more video formats
-Home theater gaming
-Listening to internet radio
-Watcing internet television
-Managing multizone audio
-Viewing/Organizing photographs
-Recording/Viewing HD video via HDMI
-Recording/DACing audio via S/PDIF
-Controlling equipment that uses IR (TVs, projectors, receivers, etc.)
-Turning lights on/off, locking/unlocking doors, opening/closing curtians
-Karaoke
-Home theater web browsing
-Internet video chat
-Acting as a Jukebox
-Emulating arcade games
-Saving money!!! (an HTPC combines all these features in one machine, often costing less money compared to separate or proprietary equipment)
-Etc.
Some of these require special hardware that is not found in a normal computer, but others just need certain software, some of which is free. If these features interest you, an HTPC may be right for you.