When upgrading from an old, rear projection tv, how much of a difference does it make to get a 4K tv or a 1080P tv? Also how important is it that my TV is "smart"?

I finally got a good job and am trying to save up money to replace the old TV that I found and fixed up. It's a Hitachi 50V500 and although it works fine, I want to upgrade to something that has a better resolution and is not so bulky in the room (and has HDMI ports).

I play quite a few games on various systems. I've got a SteamLink hooked up (using an HDMI audio extractor and an HDMI to DVI cable), Xbox 360 and original, an N64, and a Raspberry Pi (soon to add a Wii U to that collection.)

I'm not looking for a suggestion for a specific TV or even a brand, just more general information.

I don't have cable or anything, so I don't know how those things would look on a 4k tv. I also don't know if I'd need it, I just want a clear picture (I think the Hitachi is 720p and is fairly clear when it works right).

I appreciate the help in advance!

Submitted by Mr_Conelrad Xbox 720 Release Date 2015