First thing is you won't get 6.12Ghz.  You will get 3.06Ghz that can run two things at once.  It will be fast though.

The best dual mobos have 3 RAM slots for each cpu.  You put say 512 MB DDR in each of those, giving you 512 for each cpu and 1Gb overall.  You need a powerful PSU, an Antec 500w/450w should do it. 

The rest of the parts are just as normal.  Except for the addition of a few extra cooling fans, weaker ones at the front and stronger ones at the back. 

You need atleast win2000/ Unix/ xp/ 2003 operating systems for them to be affective. 

The dual mobo will be expensive.  Also i would recommend not using dual P4s, use dual Itanium or dual opteron instead.  These are optimized to run as dual processors.

I have also been thinking the same.  If you hang around for a few years, amd will have extended their ace new chip the fab and it will capable of going into the Thz.  A dual fab and whatever the top graphics card is then and a tonne of RAM etc...

It would be so fast.  I want to make it good so i am going to wait until i have done Uni and got myself financially stable enough to put £3k on a m/f of a rig.

Then i will come back on here and just look everyone else and sit back in my chair and snigger to myself.  

You can actually get 4 cpu systems but they are server only, no graphics slot etc.

Yeah because you skip the middle man.  You can build a comp for half the price you can get them from PCworld.  Plus you get valuable experience, and it looks good on application forms for stuff.  I always build.  I like the larger choice you get, the cheaper price and the ability to choose what you want in your rig.