This is my first time building a complete device, and I'm trying to figure out which kind of Arduino board to use. My requirements are USB-compatible, have an I2C interface, be able to control multiple displays, relatively cheap, and easy to program (I have learned C/C++ though). Once I'm done programming it, it will be sealed in a box roughly 6 inches x 12 inches x 12 inches, and will be run off of a 12V DC source. I also want to be able to record some measurements, and use that as a baseline for future output, but that just migt be a programming issue.
To start with I would just get an Uno, unless you needs loads of pins then get a Mega.
If cost is an issue get a Duemilanove or Duemilanove clone.
You will probably find yourself accumulating a good collection of boards over time, but the Uno is, I think, the best place to start.
Thanks, I was eyeballing that one, but also saw the Nano and some even more stripped down boards from some of the distributers. It sounds like it would be easier using the Uno though, since I am just starting, and in case I need to use any bells and whistles that the Uno has that the others don't.
One quick note: if you get Duemilanove (older one, but still available on e-bay), after you finish your project you can turn it into a ISP programmer (using ArduinoISP sketch) and put bare ATmega MCU (+ a couple of very cheap components) into your finished device.
This will work if you don't need USB in finished device. I personally use Arduino as prototyping platform and for finished devices I use bare controllers.
I've found the Mega and the SeedStudio Mega clones to be GREAT!
4 UARTs! No messing around with software serial, more IO than you can shake a digital stick at!
For really simple stuff, I bought 10 ATMEGA 328's with the Arduino boot loader, they cost me AUS$6 + crystal + a few other components, total cost was less than AUS$20.
I was messing around with my Duemilanova for ages trying to get soft serial to work, I had a GPS module, XBee modem, serial LCD display and found it frustrating.
As soon as I got my MEGA all 3 devices just worked!
It really depends on what present and future projects you have in mind.
I would think that someone that wants to create permanent projects one would need any Arduino board the uses the mega328 chip mounted in a DIP socket. That way you can develop/debug on the board and when you are done pull the 328 and build it into your project as a standalone. That's one of the best parts of the Arduino platform, you are not tied to just using a given development board for your project, you can spin off the basic processor and just purchase a new 328 chip for $6 and move on to the next development project.
However I do still love my Seeeduino mega board for all it's pins, and serial ports, and memory. So the answer is there is no such thing as being happy with just one arduino board. XD
I build prominis into my projects (bought 20 of them), after testing/coding with a deumilanova and breadboards.
Altho I might have gone with ardweeny(s) or RBBBs if I had known about them sooner.
I don't use the onboard regulators, I have 5V swallwart upplies as I need more current. In one case, I bring in 5V and boost it up to 12V with a stepup regulator, in other cases I use an offboard 7805 to knock it down and take the 5V to multiple places.
My breadboard; was really stuffed once with 8 seven segment displays & drivers before I settled on one of several maxim multiplexing parts.
CrossRoads, trying to understand your circuit is hopeless 
I suggest UNO or Duemilanove. Easy to use and still powerful enough. If you want to write very long programs, you should use Mega. From my personal experience, every 100 lines of code takes 1K of code space. I had a 2,700-line project and it took 27K of flash.
Whatever do you mean liudr?
The jumper wires seem to be going into the 4th dimension or something 
Just kidding. I can't possibly do that many jumper wires on my board. My best try was the GLCD with a few other parts, around 30 wires. 
Is this a part of your fencing scoring system?
Yes it was. In the middle is the 7406 that drives signals out to the weapons, and on the right is the shift register that drives the 6 score lights, and a 2nd one to drive a display showing the weapon selected (E, F, S, on a 7 segment display, but just LEDs here I think, 1 for each weapon just to show the logic worked that switched between them, or maybe it was 2 on for epee, and 1 or the other for foil and sabre).
Getting the protoboards and arduino all secured to the same surface really helped a lot with wiring stability. Before that I had the arduinon and a single protoboard sliding all over my laptop keyboard trying to wire as I read schematics on the screen - move anything and stuff would stop working. This works out much better, the only thing flopping around is the USB cable now, and the clipboard clip helps hold wires as well.
G'night all!
Arduino boards and their uses
Duemalinove/Uno: the "regular" standard arduino board, good for most projects
Arduino pro mini: Good price point and small size makes it great for embedded projects
Arduino Mega: Larger and more expensive with better capabilities, should usually only be used when project exceeds the capabilities of duemalonive/uno