If your going to mess about with white LED's just buy a single LED that will perform better than your 50 cheap chinese 5mm devices.
Trying to connect 50 separate LED's, house them, get rid of the heat and align them wont be fun!
The most common of the power LED's are the Luxeon products they range from 1 > 5.6 watts for a single LED and if you order the STAR type they have a nice little aluminium pad that can be used to glue or bolt the device to a heatsink. The newest they sell is called the K2 but are just becoming available and produce around 40 lumens per watts maximum at low currents, although this will drop massively as your push the device harder max continous is 1.5Amps.
The most efficient (efficatious) LED currently is the CREE Xlamp it is similar to the Luxeon with a lower power rating. The current 1 watt Xlamp will give out more light than the Luxeon 3 watt part! It produces a wopping 46 lumens per watt driven an around 60 lumens at its rated 350mA.
Osram do some good LED's the best being the OSTAR which is a monster 14watt part! but the price is prohibitive. They do a RGB version destined for mobile phone and mini projectors. The more affordable part is the
There are many similar devices and some nice chinese copies and similars but some are quite useless.
If you look about there is also a nice selection of optics available for the various LED's many can be found on ebay.
I pay £2.20 for a Luxeon 3 device and about £2 for an Xlamp but some shops sell them for upto £13!!!!
If your using a single LED you dont want to be using 12v as the power source or you will be loosing 2/3+ of the power as heat unless you use a switcher. 6v would be a better choice.
If you want any help with a simple circuit to drive them / it just ask
Phil > sorry for babbling!