ok ill assume a couple of things here (and i mean assume. there are lots of variables to consider)
1. you have a 200amp service that has available slots in the panel, residential setting
2. because you have a 200a panel in a residential setting with available slots either:
(a) you have a small house
(b) your not pulling the full load
(c) house is brand new and the slots are code required for future expansion
breaker sizes do not typically reflect the amount of power that is actually being used.
they are sized according to the wire that is connected to them, not the load. you can have 42 20amp breakers in a panel with a 200 amp main breaker..... (thats 840 amps)
what YOU need to know however is how much power your using right now. because what you dont want to happen is to exceed the 200a. then your main breaker trips and thats a pain in the ass!
just from what youve told me however i would say your good to go in your particular panel. i say this because in my experience, you could probably run every appliance in your house, turn on every light, anything that uses power, and you probably going to be pulling no more than 125amps...thats just my experience speaking tho.
if you have doubts tho always consult a pro in person
a nasty trick to pull to achieve this is to look in the phonebook and find an electrical contractor that does free estimates. when he shows up ask him to quote you a price on what you need, and have him explain the work that is needed to be done
then just never have them start the job