A cardiologist needs 4 years of med school, 3 years of residency in internal medicine, and 3-4 years of fellowship in cardiology, and you will be one in approximately 11 years. Depending on, of course, whether you are accepted into med school first of all.
Residents makes around 35K depending on where you are; fellows get paid a little more, about 45K, again depending on where you are, some places pays up to 65K for a fellow. Fellowship is not as stressful as residency (you have a team of rotating residents to yield orders at), unless if you are an oncologist, then you are almost a semi-social worker.
Fellowships in general may be less stressful than residency but cardiology is probably the execption. From what I have seen those guys work just as hard if not harder than IM residents, with hours like 7am to 7pm and then when on call they actually end up being in the hospital most of the night as opposed to say Endocrine fellows who can deal with most everything over the phone.
I don't think fellowships necessarily pay more than residencies. While I am not completely certain for cardiology fellowships, I've found this to be the case at several ophthalmology fellowships.
For example, at Bascom Palmer Eye Institute, PGY-2 (1st year) residents are paid $39,606. The salary increases a little each year so that by PGY-4 (3rd year), a resident would be paid $43,328.
Some fellowships require 1 yr of research, and the PI of the research lab picks up a portion of the fellow's salary in addition to the clinical salary; and it also varies from institution to institution. Most fellows have families; so 30K a year is on the low side of the spectrum, I would think. At Hopkins and Vandy, it is about 40-45K for an oncology fellow (which is considered low), not so sure about cardiology.