One thing you sould note about very larger resistors (10E9 and up) is
the effect of distributed capacitance on the resistor. If the stray
capacitance on a 10E9 ohm resistor is 30 fempto farads (30E-15), the
rolloff frequency is about 5KHz. If bandwidth is not a problem, no big
deal, but it is something to keep in mind.
-Jim