Scott Smith wrote:
(snip) I currently use several
computers, (2) laser printers, plus assorted peripherals. I need to power
these as well as room lighting and general use outlets.

(snip) I have never added a breaker, but as I see it - as long as I
turn off the main breaker, avoid the supply lines, and properly size the
circuits, I should be fine. (The house is about 30 years old. Box is 200A.
I assume the electric service to be original.)

Q#1. Are my assumptions correct?
Q#2. How should I split the circuits?
Q#3. How do I go about sizing (amperage) the circuits?

Thanks!

Q1. If you're far from the limit of your power supply then you're
probably fine.
Q2. It depends, if your house has anything close to modern electrical
panel then you can just plug in the circuit breaker (if there are empty
slots). Otherwise, I'd call in an electrician.
Q3. by the formula I=P/V, add all the peak wattage together then divide
it by 110 to get your amperage. Make sure you have some room to spare.

Usually there is only one circuit for the entire basement. I'd suggest
put your outlets on a different circuit and connect your lights on the
original circuit.

FC