Need help on setting up project.

I am looking to buy an Arduino unit but am not sure which one to purchase or exactly what extra parts I will need. I will be using this system on a motorcycle (12V) to control a servo which will open or close a valve a certain amount based on a given rpm input.
Please excuse my ignorance on this subject as this will be my first project of this kind.
Thank you in advance for your advice and help.

Well for the board selection I would offer the standard Uno board as the best to start with.

Lefty

I think the most important part of your project will be finding the right way to detect RPM. Unless you already have a solution you may look at this http://playground.arduino.cc/Main/ReadingRPM, to get an idea of the capabilities (note it's just one of the possible solutions). I'm pretty sure that reliability will be a major requirement in your project, so you may want to provide details about the valve as well, how you are planning to connect it to the servo (directly or through a wire), the force/torque required from the servo, whether you expect fast or smooth movements.

The ignition system I am using has a designated tach output wire. I would direct wire this to the Arduino unit. The servo will be directly mounted to the valve assembly. This will be away from the Arduino unit and connected with wire. I am not yet sure on the exact specifications of the servo I am going to use, however, I am not anticipating much more than a remote-control car steering servo with a high-torque slower moving action.

With this added information, do you still suggest the Uno as the best platform to start with? What is the simplest board unit available? I need it to be very small. It will only have the one input rpm signal and have two output wires connected to the servo. These two will alternate between power and ground to cause the servo to rotate in forward or reverse. Any thoughts?

Well there are various AVR processors that are smaller, but you may have to build the infrastructure to download code, add the crystals, etc. that are normally part of the development board, but if you want a small package, you would need to build them yourself. However, if you are a beginner, it may be beyond your current skill set to make your own board. Here are some directions: http://arduino.cc/en/Main/Standalone

The Arduino Nano board might be simpler, in that it has all of the bits and pieces, and you would just put it on a small perfboard (or breadboard): http://arduino.cc/en/Main/ArduinoBoardNano

Going outside of the official Arduino boards, some boards to think about include. You probably should go to the forums the vendors provide for any help, and copies of the tool chains:

Thank you very much for your reply. I will some more research into that and reply back with any further questions. Again, thank you all for your help!

Do they make any covers for the Nano or any other Arduino unit?

reactionracing: Do they make any covers for the Nano or any other Arduino unit?

I'm not sure what you mean by covers. If you mean enclosures, there are various enclosures meant for the Uno and other development boards. One cute one is the Adafruit Menta that has an Arduino compatible processor with prototype board that fits in a tin the same size as Altoids tin (they include a non-labeled tin). You would need to drill holes for the sensor wires and power, and add a FDTI cable to program the processor. Note on a motorcycle, you likely need to deal with weather sealing: http://www.adafruit.com/products/795