ATTENTION! We need help starting up!

Does he not make them any more?
Then why does he still have the buy option on his website.
Good job I have yet to order one.

Yeah, the midines situation is a nightmare still. Essays could be written on the subject.

Here's rough information on all the consoles you have - I'm not hunting down a ton of links, but all these names are googleable and should get you started.

There are a few different ways to write music on the NES, but other than using a midines, they mostly involve getting a powerpak flash cart to put them on. An alternative carts is currently in beta being made by arfink (there are threads about it on this board) which is going to be awesome, but the powerpak is a great product that is available now.

From there you're basically going to be working with trackers. NTRQ and Pulsar are native to the NES, and famitracker runs on your computer and outputs .nsf files that you can run from the powerpak.

There's a few cool programs for just playing the NES using the NES controllers themselves.

The game boys you have are going to be very useful. They can all run LSDJ (a tracker), Nanoloop (1.x for the classic gameboys or 2.x for the advance or later) or mGB (which is similar in function to midines) but they will all sound somewhat different. There's a lot written about the differences already, but you're best off experimenting.

All of these pieces of software also need flashcarts. There's a lot of drama behind the history of the production of these carts, but the EMS 64MB USB carts are the steady workhorses that everyone has. You'll need a windows computer to transfer to/from. There are other types of carts worth checking out.

If you want to get MIDI into the gameboys, your best bet is through an arduinoboy, an interface designed by trash80 (who also wrote mGB). Your keytar will be able to plug into one directly, or you can control it straight through USB on the arduino. The arduinoboy is very easy to build (with an arduino, obviously), and people occasionally sell them.

There's also software called Pro Performer - a few different versions - which is harder to find and doesn't work on flash carts. It focuses on playing music with the game boy's own buttons. There's also a japanese rom called muddyGB which works similarly but is super noisy.

The SP needs a headphone adapter to get audio out.

Your N64 is basically useless, nothing really exists on it for making music.

The gamecube can run all the gameboy stuff with a gameboy adapter.

You should be able to buy all the stuff you need between these three websites, which are all US based. My understanding is that it's hard to get some of this stuff new in the UK, so if you're getting a lot, you're probably best off just having it shipped in as big of a pile as possible. Maybe I'm wrong and someone else in the UK will correct me? But I think that's the case.
http://store.kitsch-bent.com/
http://nonelectronics.com/
http://www.retrousb.com/index.php (ONLY place to get the powerpak cart)

Thanks for the fast reply's guys.
So im thinking the first thing I will try to do is convert a Gameboy into one of these Arduinoboys, which after a little Googling looks quite simple. Good job too having a background in electronics.
So I understand if I order a MidiNes, although it states on the website that it will take 1 - 3 Months its more like 2 years? That sucks, the MidiNes was the big selling point for me. I presume you can get them second hand on E-bay every so often then?

Honestly, if you order a midines from x|k, it may never show up. I bought mine on craigslist.

Yeah, the arduinoboy build is pretty simple, as long as you have a link cable with a pin 1 for power, or don't mind using an external power source (or can figure out how to move one of the unused pins, as I have done). Normally it's built as a standalone device which hangs like a tail from your gameboy, but it can be built into a DMG using an arduino pro mini.

There was a guy at PAX this weekend who talked to me for a while about some sort of NEW midi NES project he created and wondered where to promote it.  I gave him the address for this website and 8bc, so hopefully he'll pop up soon.

I bet you it's FaMi, the Batsly Adams program that's on the way - he's been busy with viz and the DrunkenNES project, but hopefully it'll be done soon. Look him up at batslyadams.com.