I have a '90 Honda Accord with about 155,000 miles on it, and recently
it started making a disturbing noise. I personally think it's the outer
CV joint on the left-front wheel, but I'm not sure, so I figured I
would describe my problem and ask for help.

There are basically two symptoms.
1) If the car is taken on a long trip it will start to make a very
quiet thumping noise at high speed, as the trip goes on the thump will
get louder and louder to the point where it can be felt a little in the
steering wheel. The noise is dependent on where the gas it. In the
initial stages having my foot off the gas, or down a fair way will make
it go away. At all stages pushing the clutch in will make the noise go
away (Would a bearing do that? I figure the lack of force on the CV
joint makes the problem go away).
If the car is stopped (for gas or food or something), the noise goes
away, but it'll go through the stages again. For short stops it doesn't
take that long to get back where it was.
Sometimes the noise will just stop at random, and again start to work
it's way through the stages.
2) Sometimes when the car is making a very sharp right turn (as sharp
as it can) it'll make a few very loud, regularly spaced, noises; these
are similar to the noise the car makes under situation 1 when it has
been driven for a long time.

So my questions are:
Is it a CV joint, a Bearing, or something else?
How much should it cost to get repaired?
How hard would it be to fix myself?
Is this a "keep the car off the road" sort of problem? Or a "pay
attention incase it gets worse" sort of problem?