> I'm assuming I need a heat sink

Not necessarily. Use a series resistor between wallwart +ve and
7805 i/p. This will take the brunt of the voltage drop and heat up
instead of the regulator. Something around 15-22 ohms should do,
preferably not a carbon for longer life. Choose so that voltage at
7805 is still up around 8-ish volts. Probably need a 2W, you'll have
work that out from actual current passed. With the figures you gave
it looks like the 7805 is generating about 0.5W heat (assuming,
optimistically, actual 12V), with the added bonus that it's already at
120F. With a dropping resistor you should be able to keep the
7805 in that region

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist