Ehh. Depends, there can be some artifacts. The input mode is a little weird so it's not necessarily well supported on every set. It's not going to be 100% perfect because the NES pixels aren't square. If ultra clean video is your priority may as well play on an emulator like FCEUX, if you set the scaling settings correctly you'll get a fairly accurate per pixel look.
As for video output, I'm not 100 percent sure what you're saying. When you say sweet ass video cables I assume you mean composite out? The American NES already had that. I don't think the NES could be modded with anything better, I remember reading that people would hunt the arcade version of the NES (one of those select a game coin-op deals) so they could get RGB output from NES games.
Just google the name of the system you are looking and you'll surely find discussions on the topic, e.g. sega genesis video mod, Atari video mod... whatever.
I'd like to get my consoles modded for superior video output, but it seems in the US we have a tougher time than in Europe. Over there RGB over SCART is pretty common, so all you have to do is get RGB from your console, which many of them do... but nothing will output in component, which is the high-end analog input type in the US.
And yes, there are people that will do the video mods for money. Check out this ebay auction