3575H downcoverts but still pretty good
When any digital signal (HD or SD) is received, I think it displays it in 480p. HD shows still look pretty good but they are not HD. The literature calls it an SDTV tuner so I don't expect HD. When hooked up via HDMI, the manual says that the image is upconverted to 1080p. However, I don't know if this only applies to the DVD and HDD recordings. I must confess that I am pretty new to the HD world. My eye for seeing differences between 720p, 1080i, 1080p (native or upconverted) is not very good.
The 3575H keeps analog and digital tv channels separately. There is a toggle button that switches between analog and digital channels. This is in contrast to my Panasonic Plasma that also has both NTSC and ATSC tuners. My tv tuner will comingle my analog and digital channels. Hence, on my tv, channel 5 will be CBC analog and the next channel, 5.1, will be CBC digital HD. On the 3575H, channel 5 will be with the analog channels. Then when I press the tv/dtv button, I would get channel 5.1 with the digital channels.
I realise that I won't be able to record HD but it's a sacrifice I'm willing to make. I don't want to pay Rogers and the HTPC is way too fiddly. My wife would never want to deal with an HTPC. And forget about TIVO HD $$$$. Hence, if I want to watch HD, I'll watch it live on the Panasonic plasma. Everything else will be SD or SD upcoverted to 1080p.
Since you almost never watch live TV, the 3575H may not what you want if you also want HD. Since you've got the HTPC working, can't you hook up your HTPC to your projector?