Well I did this awhile ago on another xbox but never wrote a tutorial just posted to ask questions.
So here the tutorial is.First off, This will void your xbox warranty and if you stuff anything up its your own fault and I take no responsibility. These instructions and pictures are to be used as a guide only.
If you agree to that now open up your xbox and a memory unit (256/512mb doesnt matter) Theres lots of instructions for this around.
Next you will need a few tools.
5 pieces of small wire about 20cm long (got mine out of a usb cable)
Next you want to strip off the plug on the memory unit. I do this by taking wire cutters and cutting the legs.
The legs on the side you cant get at but you can just pull them off the plastic plug.
Then you can pull the plastic bit off its only got some weak glue and two plastic legs on the bottom.
Here is a pin out once the plug is removed.
You can ether trim down the rest of the metal legs or de solder them.
Next you want to attach the 5 wires. First off bride all the earth's. Two pins on each side you can just use solder to bride them. Then take one of your 5 wires and solder it to one side of the bridged earth's.
Trim it down a bit so its not to long and solder the other side to the bridged earth's.
Now solder the rest take note of wire colours if you are using different coloured wires, This just makes it easier when soldering to mother board. You only need to use 1 wire for the earth to the mother board since they are all bridged together.
Once that's done put some electrical tape around the memory unit, (its best if you cover the whole thing so it wont short out on the xbox and holds the wires in place better I only had a little bit of tape left so that's why I didn't use much).
Now here is the pin out for the motherboard side.
Its easy to tell which one is the 3.3v due to it have a little resistor thing, The out side pins are earth's
Now put the wires through the hole and solder them to the correct pins, If you have flux use it these points can be hard to get solder to stick to.
How I do it is put some extra solder on the pins then see if I can scrape it off with a finger nail. (That it is lead free solder you will need to hold your soldering iron on it longer to get the stuff on the motherboard to melt and join with your added solder.)
Then add some solder to the wire and its quite easy from there. Mine isn't the cleanest job but it works.
Now check that none of the points are joined so they don't short out.
Now put some tape down over the soldered points so it doesnt short out on the xbox case. Also good idea to tape the wires coming in the hole once you have adjusted them so there isnt to much slack. This will stop any pull on your solder points.Didnt take a pic since its easy enough to under stand.
Now back to the top side. make sure the wires are run in a good place to stop them touching the heat sink. You dont want that to melt the wires coating and short them. I put them between a cap and the memory unit plug seems to work good holding them in place.
Now get some double sided tape or fold some electrical tape over on its self and stick the memory unit to the hdd plug.
Now one last check for anything shorting before you carefully put it back in its metal case.
plug the fan and ring of light board back in and power on test it. (don't leave it running to long with out shroud)
Basically a good test is go to settings/memory then format the device. If that works then it should work for reading and writing saves.
(Iv renamed my memory unit to Internal)
If it isn't working check back over your wires and solder points.
Now stick the short screws back in to hold the mother board in place, And start to reassemble the xbox.
When you come to screwing the top on with the long screws remember not to put one in the hole with the memory unit wires.
And that's that you have your self a xbox with internal memory unit.
If you stick a memory card in slot B (the one the internal is soldered to) it defaults to the one that responds fastest. (one in the slot due to no lag from the longer wires).
But its best just not to use the slot I don't know the long term impact from having 2 memory units getting powered from same plug, could burn something out.