The rule of thumb is to load a circuit no more than 80% of capacity. This means that for a 20 AMP circuit, the total load should not exceed 16Amps. Running more than that can cause the breaker to fail over time. This failure will show up as one of 2 options:
1) The breaker will trip with less than 20A total
2) The breaker won't trip EVER.

Both cases are cause for concern.

In regards to replacing a 15A breaker with a 20A one, this is a violation of the electrical code. The wires are sized based upon the load they will carry.

For a 15A circuit, the wire size is 14 guage. A 20 Circuit will use a 12. Placing a 20A breaker with 14G wire can cause hte wire to heat up and possible cause a fire. Don't do it.

Dan