Buying a new bike for a newbie

Okay, the last time I bought a bike (Centurian 10-speed in 1982 @ 12 yrs
old) the choice was between the Centurian and either a Mongoose or
Diamondback BMX bike (or a CFX I believe). Anyway, I just took my 4 year
old son in to get his first bike (a simple Trek 16" Jet) and I was thinking
about getting myself a bike, but I am lost as to what to get. I am hoping
for some recommendations based on the following probable use
characteristics:

I live in Portland, OR and we do have hills....

I will be riding on the street/sidewalk/hard trail most likely 80-90% of the
time, but may venture a little more off-road once in a while.

I want something 'light'

I would like to stay under $500.

I think Disk brakes are cool, but do I need them????

I like to go fast, but comfort is also important....

Thats what I have thought about so far, any help?

Thanks for your time.


K