I'm something of a noob with electronics but I have a small project I want to do.
A few months ago my mum broke her snake head lamp and is talking about spending a few hundred bucks to replace it but I reckon I could probably do something a hell of a lot cheaper. The light's physical structure is still good, its just the PSU that's busted so I was thinking that I might make an LED array running off a 12v transformer as a replacement.
I came up with the highly technical diagram I've attached, would this sort of design work? I'm not sure how the half power switch would work, could it underpower all the LEDs in the array and let them output dimmer light or would it be better to wire it so it lights 2 of the strips on half and all 4 on full?

This site may be of some help. There are several pages of info besides the handy wizard . . . =)

Maybe this site will help

Is that a switching power supply or just a cheep unregulated one. The problem with the latter is that they only run at the advertised voltage when operating at the marked load. So if you connect a few LED's to it they will not be drawing nearly enough current causing the power supply to output something along the lines of 30v causing your LED's to burn out.
As for the the LED array, the link that Stuff posted works well. Also it can be easy to piece them out in your head. If you have 3v LED's you can connect 4 of them in series to make 12v. Throw a 1 ohm resister in line with that to prevent pulling too much current thru em.
Also if you need led's I would recommend abctronics.com They have a scary website but they are cheep and I buy from them all the time.
For resisters and powersupplies allelectronics.com is a good chioce