I have a 10W RGB LED that you are trying to use. These are exact voltage drops of the LED.
Red : 6.39V
Green : 9.38V
Blue : 9.22V
All at 350mA
If you want to make your own LED driver, why don't you use my design which is exactly fit with what you want to do. I published all Eagle cad files. http://code.google.com/p/neuroelec/source/browse/#svn%2Ftrunk%2FHP_RGB_LED_Shield
If you design switching power first time, there are a lot of things to think about it carefully. The LED driver part of my design will cost less than $30 for 3 channels. You will have very efficient drivers and easy of PWM control that are proven to work. I will send you exact manufacture part numbers and BOM, if you need them. You will only need three drivers to drive 9x 10W RGB LED with 36V power supply such as this. http://www.powersupplydepot.com/productview.asp?product=16034+PS
If you don't want to or don't need to make your own driver, I would shamelessly recommend my RGB LED shield.
You can use rest of your budget for led heat sink and housing.
3 x RGB shields : $37 x 3 = $111 + $3 shipping
9 x 10W RGB LED : $11.49 x 9 = $103.41 free shipping
36V/4.2A Power = $40.75 + $11 shipping
4 x RGB shields : $37 x 4 = $148 + $3 shipping
8 x 10W RGB LED : $11.49 x 8 = $91.92 free shipping
24V/5A Power = $19.33 free shipping
@neurostar: "In theory, driving power LEDs in parallel is a bad idea." The design being worked from the first page of the thread had current limit resistors in each string, thus they are not actually in parallel.
I was talking about his initial idea. Even though it is not ideal, in practice you can drive leds in parallel with same leds which is the case for his setup. I tried, it worked with +-5% current difference though.