Components have data sheets, finished products usually don't - they have manuals and schematics. The web site is the manual for the arduino. Check the arduino hardware page http://www.arduino.cc/en/Main/Hardware and it'll give you some useful information. For example, the schematic will reveal that nearly all of the arduino's pins are simply wired directly to the Atmel's pins. Check the ATmega8 data sheet for signal levels and input voltage specs.
For input voltage to the board, the 5V input expects a regulated 5VDC. The arduino NG normally gets this from USB. The "9V" connectors are tied to the input of the 7805 (78M05 to be precise, on my arduino NG), so you'll need to check the datasheet for a the 7805 to find that. I think the 78xx series need 2V over the output to properly regulate, so the 7805 need a minimum of 7VDC input, and a maximum of around 35VDC input. Almost any DC output power supply over 7V you're likely to find in your junk box should be OK.
On p245 of the ATmega8 data sheet they give electrical ratings for the inputs. a High is a minimum of 0.7*Vcc (3.5V on a 5V arduino) and a maximum of Vcc+0.5V. The absolute max ratings of the ATmega8 is 5.5V.
-j