Sorry, I thought what I wanted was simple, but looks it is much more complicated than I thought.
I cant understand half of what you are saying, and this is MY fault, I know.

I thought a simple resistor and driver would solve my problem.

What I really want is two things:

First: Drive this 3W RGB Led from a microcontroller pin, preferably using a cheap and simple way. The ULN is a nice option, but it didnt worked well as I said.

Second: I want to drive devices, as this controller will drive many type of devices, voltage and current will vary every time (place to place). I want something with a maximum current of something like 10A, so looking around I bumped in MOSFET IRF640 which is powerful, easy to find and not so expensive, now I'm searching for a simple way to conect it to the 8051 output. I think switching the ground is better because I will be free to choose the driven devices voltages.
http://img.mercadolivre.com.br/jm/img?s=...13.jpg&v=O
http://img.mercadolivre.com.br/jm/img?s=...25.jpg&v=O
In this two pictures shows a microcontrolled device, I asked the seller for the output transisotr, but he didnt said to me, he said only it was a N channel MOSFET with more than 10A. I was looking for the pictures, looks it hasnt much components between the microcontroller and the MOSFETs, how he done this?

Thanks.