I have an iScan Duo, similar to the Edge. It largely depends how your TV handles the feed from the cable box, which is why I'd suggest getting your TV first and then seeing what needs improving. I don't watch much streaming content, but the Edge would be adequate at best; there's only so much you can do with heavily compressed sources, and the chip inside te Edge/Duo isn't renowned for its noise reduction. It does deinterlace and scale well (especially for its price), which may help with cable channels, but again, how your TV handles the feed from the cable box will decide if its really necessary.
If your cable box has an HDMI output, you'd just plug that and your blu-ray player into the Edge (if you decide to get one)and then an HDMI cable from the Edge to the TV. If you want to take advantage of higher quality/surround sound audio, and not just your TV speakers, you'll need an amp.