It all depends on where your cardiology fellowship is....
Most places with fellowships pay a graduated rate based on PGY year level. So at one your pay rate will be at a the PGY-4 level.
Some programs, such as inner city, or prestigous programs often started their PGY-1 grads at $36,000 and each year goes up around 2-3%, so a PGY-4 at this program would probably make around $41-43,000.
So, again this all depends on where your fellowship is, if you are staying right at the same place you are doing your residency at, you will make a slight increase, just as you received going from intern to 2nd year or 2nd year to third year.
Now what can make a difference is ability to moonlight...most programs allow this, my take on this is that it still does not pay enough to get me to do it..