Generally, OEM brakes on your car are fine, but if you're adding power, then you'll need brakes that can handle the power. For example, if you have a 9" factory rotor with standard calipers, but upgrade your car to add 150 horsepower, you'll want to get bigger rotors with better calipers. This, of course is assuming you'll be driving it faster as well. Upgraded brakes will help not only in the safety front, but also racing. They help stop much faster, and won't fade nearly as quick.

Price on upgraded brakes vary wildly. On my 300ZX for example, I could simply buy drilled & slotted rotors and leave it at that, and it would cost only the price of the rotors. But they also sell brake kits with new rotors, calipers, brake lines, and the whole system for a few thousand dollars. I even see some that make you get bigger wheels because the rotors won't fit inside the stock wheel.

Also, how much will a set of good brakes set you back? From what I've heard they are rather expensive.