I am a newbie - haven't touched the inside of electronic stuff since high school shop classes. I have been riding ebikes for a while but would like to take a shot at making my own to meet very particular requirements. The most unique one will be the inclusion of a small gasoline powered motor on it to give it honking big range 
In the near future I will be seeking info on how/where to buy sensors to measure and access speed, temperature, voltage, oil pressure, angle, acceleration, gps info and motion and then how to control relays and a few servos, as well as send output to an lcd touchscreen lcd. I noticed in the store that this place sells many of the sensors and servos that I will need - am really thankful for that! My biggest concern is what to use to control them all - that I am a bit puzzles about. I have seen mini itx mobos - about 5"by 6" for under $100, basically the guts from a netbook - that look like they would do the trick, but would have to use a solid state hard drive or cf card in them. I wonder if there is a playstation or some other such device taht would also do it? Anyway, that is my "dream machine" that I will be working on over the next 6 months or so. Nice to meet you...and, by the way, I live in China now teaching English here. Makes it easy to have access to a lot of parts but difficult to find quality 
It is here somewhere, just often really difficult to find.
For what you want to do, I would start simple and go with an UNO or similar (since you haven't touched electronics in a while); then, once you get that understood (and maybe have integrated a few sensors and/or servos and such), you can decide on how to set things up for your hybrid bike project. Based on what you've mentioned, its possible you could stick with an UNO (or other ATMega328 -based controller), but depending on the amount of code for your project (and your coding skills), you might find a Mega or Sanguino type controller (ie, 1280, 2560 or 644) a better fit. I don't think you would need to go all the way to a PC motherboard (like the mini-ITX boards you mention), simply because I can't imagine any kind of function you are trying to do that would need that kind of power. Then again, I don't know all the specifics of the project. If you think you might need it, for some reason, then set up such a board as your Arduino dev system (you'd need it anyhow) - then your Arduino would simply be the interface board, via USB, for sensor and servo I/O with the Mini ITX -based PC. If you find you don't need it, then you end up with a mobo for some other cool project, or you could set it up as an on-board bike PC of some sort...