24 LEDs max for a 29G. I used a 23"x4" heatsink from rapidled, but I really like their black 20"x6" and I would pick that one today with the splash shield and fans.
If you plan on putting the light right over the water like I did, and not use optics, then dimmable isn't a must have feature for what you want to do. If you were to hang the light a little higher and use optics, then I would say having dimming ability would be more important because you would probably want to turn them down. When you add optics it usually doubles the intensity. I really like the Typhoon controller from boostled for DIY lights, the DIM4 controller from ledgroupbuy is nice as well, but a little more expensive.
Personally, as far as LED choices today, I would pick 4 cool white, 2 warm white, 6 violet (make sure it is in the 410-430nm range), 8 royal blue, and 2 regular blue, no optics, and put the fixture no more than 6" off the water. The cheapest route without dimming, I would put the whites and violet LEDs on 1 LPC-35-700 driver, for 2 reasons, first is the Violet leds on the market today can only handle 700mA and second is you really don't want or need to run the whites higher than that. Whites are really bright and quickly wash out all the other colors which is why I think 6 whites total at 50% max power (they can handle 1500mA) will still light up a 29G tank fine. I would use a LPC-60-1050 for the other string. Or if you want dimmable, 2 ELN-60-48 D or P drivers, depending on how you want to be able to dim them. D model uses knobs or the DIM4 controller, P model works with the Typhoon.
I would really recommend spending the extra and getting the ELN-60-48P drives with a Typhoon controller. The controller makes it really easy to setup timers and how long the ramp up period is. As an example, string 1, royal blue and blue string, comes on at 10am but slowly ramps up in intensity over the course of 4 hours. At 1pm, the whites and violets kick on and slowly ramp up to 70% full power so at 2pm the blue string is at 100%, and 2:30pm the white sting will hit 70%, both stay on for however many hours and then each string will independently dim down. It makes a really nice natural lighting schedule and you can enjoy your tank more because it will be lit up more, just not intensely lit during dusk and dawn. It isn't a required feature, but for an extra $100 tops it turns a "functional" LED light into a much more feature rich light. No need to worry about timers, the controller handles it. You can even run fans off the controller so you don't need a power supply just for the fans.
Here was when I ran 24 over a 29G.
[video=youtube;fUlInNEgVwk]http://www.youtube.com/watch?v=fUlInNEgVwk[/video]
Here is a light I made for a fellow reefer last year for a 60G cube, this had everything on the fixture, and only uses 1 power cord for everything. Has 26 LEDs for the display and 4 moonlights with the Typhoon controller. All I had to work with was a dremel and a cordless drill so it isn't perfect, but I made it for free (person provided parts) so can't really complain.
