DuaneB:
I understand that the Hex buffer will take a TTL input and sink or source a larger current and voltage on the output side, is there a common IC that does the reverse i.e. takes a 6,7,8 or more volt input and converts it to TTL on the output side, with some protection in between ? If switching speed is a priority is an opto isolator my best option ?
Not voltage typically, just current - for buffers and opto-isolators; sometimes you may find parts that will allow for switching slightly higher voltages (it depends on the transistor or mosfet outputs). Also note that many buffers are inverted - that is, you put a logic HIGH on the input, and get a logic LOW on the output. Something else to keep in mind (read the datasheets of the parts in question - always try to read the datasheet for any "active" part if you can - and passives in the cases of diodes or similar) is that a part may require that pins are pulled either high or low, and not left floating; leaving pins floating on certain parts can damage them (or cause the outputs to be wonky).
There are voltage level conversion chips that can take (most commonly) a TTL level (5 volts) and drop it down to TTL 3.3 volts; if you are just needed to drop the voltage to supply power to a part (say you have a 12 volt battery, and need to run a 5 volt motor), then using a voltage regulator (and if needing the current - a bypass transistor; or an adjustable switching regulator) to drop the voltage to run the motor (or circuit - or whatever) can be used. But these aren't typically used to switch voltage to the motor or circuit - you would typically do that -after- the regulator or whatever.
Opto-isolators are most useful for, well, isolating a circuit; say on one side of the circuit you have a beefy motor, with transistors switching a lot of current, and with possibly voltage spikes being introduced by switching. You don't want this getting to the logic side of things, because doing so might cause the logic to behave wonky, reset your microcontroller, or burn something out. So you use an opto-isolator.
Basically, all an opto-isolator is, is an LED sitting next to a photo-transistor. When the LED is lit, the photo-transistor is turned on (note - check the datasheet for the opto-isolator; some have current limit resistors already for the LED, some don't - if it doesn't then you need to check the current and voltage needed for the input/LED in the opto-isolator, then calculate using ohm's law the size of resistor needed to pass that current, so you don't burn out the LED); this photo-transistor then controls the load, or more generally switches a beefier transistor controlling the rest of the circuit.
The key, though, is that with the photo-transistor, you can have -completely- separate grounds and power inputs for the isolated circuit - so that no noise or voltage spikes will travel back into the logic control. Your logic remains safe from the other side (unless you don't pay attention to how you route wires or traces on the board, in which case induced voltages/currents can be set up and cause problems - typically very difficult to diagnose problems, as well).
Now - for this R/C car, you may or may not want the isolation; it seems like (at least in the datasheet on page 7) they already use a transistor for some amount of buffering on the circuit (parts Q4 and Q9 for the forward/reverse, and parts Q10 and Q15 for left/right); if something goes wonky, these would probably burn out first before anything happened to the Arduino. But - a buffer or similar might be a good part to add if you are paranoid. You should also check and make sure that the manufacturer of your car actually put in those transistors (do this by following the traces - a good way to do this is to make a nice scan of the bottom - which you have - then using a paint program to flood fill the traces from part to part, while looking at the top-side and comparing things; make a rough schematic while you're at it if you want - it could be helpful!).
Something else to notice on that example circuit in the datasheet (check out your car to see if it is used): Remember the turbo function (pin 12)? Notice how it is wired in the example circuit: it goes to a transistor (Q3), that then turns on the "forward" function (Q4 - which turns on the h-bridge transistors Q6 and Q7 - making the motor spin in the "forward" direction); this flow also turns on transistor Q2, which switches the full 12 volts (or whatever the upper voltage is) into the motor circuit (instead of the normal 9 volts which is tapped from the battery at the right) - giving you the "turbo boost" function; pretty neat, huh?
Also something else; check out how your car steers - based on what I can see on the circuit board, page 7 is your likely example circuit - but notice that the datasheet lists a very different circuit on page 8 (I am not sure how this is supposed to work, unless "left" and "right" go to pins 6 and 7 or something.
Another thing (always something with me - I should write a book!) - ultimately - once you get this thing working with "probing", and you start to hook in the Arduino, you are going to want to desolder the chip from the board (another reason not to solder to the chip) - this is so the chip doesn't use up any current (you want that to power the robot and the Arduino!), and also so no power is supplied to the radio receiver portion of the circuit (consuming more current, as well as possibly getting interference into the circuit - making your robot do strange things).
Finally - I want to point out that the page 7 example circuit shows the view of the IC from the top down (exactly as I described the pin numbering in my last post) - just look at that to verify things, ok?