Horse the Band are awful and not bitpop. They're retarded hardcore with occassional video game keyboards.
Almost every high end keyboard can make 8-bit-ish sounds. Fuck, even a shitty Casio has "synth lead 3" setting, which sounds exactly like an NES. If you want to do it on your computer, there's many choices.. bit boy, ymsynth, for example; peach, toad, and triforce from
www.tweakbench.com also work (if that website works still -- it probably does; if not, try searching
www.kvraudio.com for vst plugins). If you want to make it 4-bit, for your Gameboy, you should get NANOLOOP. There is an excellent compilation of established artists making Nanoloop compositions (called "nanoloop vol. 1", i'm sure it's on soulseek..), one which is by Merzbow. Guess how his sounds. (If you guessed "like a Game Boy farting static", you're correct!)
The most essential "bitpop" band is YMCK. Japanese 8-bit pop. Amazing stuff. I discovered 'em a while back, because I really love J-Pop. In fact, lots of really good J-Pop acts use video game keyboards a lot (I'm look at Perfume, Motocompo, Mac Donald Duck Eclair, etc) so if you want actual POPPY stuff, look into that. Not the J-Pop shit that your little sister likes, I'm talking about the picopop J-Pop genre (
http://en.wikipedia.org/wiki/Picopop). I'd also reccomend DAT Politics if you want some of that stuff in a cutesy, glitchy form. And Sabrepulse if you want straight-up 8-bit breakcore stuff... his nickname is "Venetian Snes" if that tells you anything. I've heard many groups like this, and it's usually VERY boring after a few songs, but ... hmm. It's okay, I guess. I prefer the "micromusic" genre, which is similiar but less annoying. I used to make music in this style, did a Dillinger Escape Plan cover, a Radiohead cover, and it translated to some music by my band... also, my friend Hara Kari Kitty did music in this style. My good friend Bubblegum Octopus kinda does music like this but he's started to incorporate other, non-video game elements in there too... It's very easy to get bored/sick of. You just can't take it very far. Also, if you don't feel like using plugins, when I did music like this (years ago) I wasn't aware of the plugins yet, so what I did was record video game music and cut out snippets of the synth and then edit it in the piano roll function of Fruity Loops. Hmm.. I'm trying to find it but I can't.. there's a youtube video of someone extracting every element of the music to Journey to Silius (my favorite NES game, also with the best music) in Fruity Loops with some plugin, leaving just the synth line, and then mutating it into a new song. Uh.. honestly, I'm not looking to deeply, you can probably find all kinds of shit on youtube, videos of people "making" video game music.
I'm sure you're aware of this:
http://www.8bitpeoples.com/discography_gfx.php
Like a hundred or so free albums of nothing but music in this style.