Sure, I'd be happy to share this project !
Please note, my project is long from finished, I've just received my Arduino Mega yesterday and a bunch of piranha leds is on their way.
But I've already figured out most things in my head.
I'll use the mosfets that were recommended here ( still have to order them ) and an external PSU to power the whole project.
As for the resistor values, I'll wait another week to order those until the LEDs arrive and I can test the correct values to use. ( Because in my case I'll need around 600 of them, so I don't want to end up havig bought the wrong ones. )
If you've got any more questions or suggestions, by all means post 'em