You don't mention which version of Media Center - assuming that you want to use extenders for HD content then you will need to be running Vista MC and have V2 extenders - or Xbox360s. Out of the box Media Center, either XP or Vista, will not allow you to use OTA HD in Canada - but that is fixed quite easily - look for the ATSC Canada hack by Peter Near.
But there is no way to get HD cable into Media Center in Canada - there is the Hauppauge HD-PVR box that 57 mentioned under "Analogue hole". This box is supposed to be shipping this week but it is unlikely to be supported by Vista MC in the near term.
Your best option therefore is to use SageTV and buy the Sage extenders. Sage is supporting the HD-PVR out of the box. Note that these Hauppauge HD-PVR devices cost $250 so you will need one for each HD signal that you want to extend through your house plus one HD cable box - ouch! If you don't need all of the extended signals to be HD then you can just by a regular analog tuner card and an SD cable box which will save several hundred $ per tuner.
Note that you can run Sage on your WHS box so this negates the need for an HTPC.
I have been running XP MCE 2005 for about 18 months with several V1 extenders through my house. I also have a WHS box with 2TB of storage. I am likely to buy at least one of these Hauppauge HD-PVR units and switch to Sage as Microsoft seems to have lost interest in the Media Center concept, especially for Canadians.