what are the differences between NC29 and NC23 such as power, shape, size?
etc etc?

jesus have a look at previous post! lazy bugger especially the one saying cbr400 specifications

this could also fall in, which one is the best and most preferred :)

well if you asked then then you would have got a different answer

depends what your budget is.

NC23 are usually a bit cheap but look a little more dated but do have better engines.

see, im 17 in december and i'm looking to sell my RS50 for a CBR400 and have cbr600 farings on it.
but ive heard that the NC29 is faster, and the NC23 is faster so i thought i'd come to best place to find out

To be totally honest if its speed your after then they are both going to be rocket ships if you have come off an RS50.
check insurance before commiting to a bike.

CBR6RR fairings can be fitted. how deep are your pockets?!?

i can afford the insurance and a bike around the £800 mark, its the cbr kit im not sure on how much it will cost to determine if i have enough money for it :)