connecting to amplifier

Unfortunately, modern macs standard audio outputs are certainly rather feeble these days. Before things went Intel they were generally better.
If you want cheap, then even a (USB) Griffin iMic is better than most macs standard outputs. probably worth a shot, as if you don;t like it and still want something better, then you wasted hardly any money (They are about £30 in the UK, so $50'ish?)

Sound quality wise, USB or Firewire is rather irrelevant. Its just a way of getting digitised audio data in and out of a computer. Its the quality of the AD/DA converters that largely makes the difference. Firewire does tend to be better however. Not so much for quality, but it is more dependable and has less latency.
Then again, if it is just for listening to music via iTunes, and you are not doing any recording, then any number of USB audio interfaces will do just fine. Although most are still geared somewhat towards music studio use.
So long as you get one that has good feedback for Mac use, then you are well on your way.
I have an Apogee (firewire) interface which is about as good as it gets. But they don't come cheap of course.
There are dozens of other reasonably priced and straightforward USB ones you could choose from though.. Perhaps have a look at brands like M-Audio, Edirol and Tascam.
Many interfaces have multiple inputs and outputs, like 8+.. All you need is stereo of course. It will have inputs too, but you'll likely not need those.

want to hear something cool? i have my itunes on my mbp and i have a 1/8" to rca cord that i plug into my mixer and it sounds GOOD. then again i have studio monitors and an 800watt amp but it seriously sounds great. don't worry about wasting money on an i/o for just listening to mp3s.