I'll add to the good advice you've gotten so far...
The Hauppauge HD-PVR is your only option at this point to get HD from your Shaw boxes. You'll need two HDPVR devices, as well as addon software called DVBSBridge.
The HDPVR records in h264, which is a newer and more efficient compression than MPEG2. However Media Center only supports h264 content in Windows 7 or in the Vista TV Pack update (go with Win7, TVPack is buggy). Additionally, since the recording is in h264 your Linksys extender won't be able to play it back. The Xbox plays it back without issue.
Bottom line for recording HD Cable is that you can do it, but it'll take a bit of work and might cost you a bit on new hardware (notably the tuners and a replacement for your Linksys).
Another option, if you're anywhere near a large city, is to look into over the air high definition. Let me know what city you live near, and I'll let you know if that's a reasonable option or not.