I believe the signal you get from the STB is 1080i, not 1080p, since all HDTV sources are 1080i. The only source for 1080p to my knowledge is Bluray. I could be out to lunch on this, and I'm sure the local "geeks" will put me in my place if I am.
NO! Buy a 1920x1080 TV. If the input signal is 1080i the TV itself will deinterlace it ( at the very end it still need a "p" signal). If the signal is 720p the TV will upconvert it to 1080p. Some TVs do this better than others (you get what you pay for...).
720P (actually 768P) vs 1080P TVs have been discussed many times in this forum - for years. This is only one spec you need to look at and several others may be more important. Since the question of SD formats has been answered, this thread will be closed as we don't wish to start, yet another 720P vs 1080P thread.
Here's but one example found by searching for "720P 1080P" in the thread title, under advanced search.
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