What exactly are you looking for? What exactly are you wanting to do?
If you are planning on having a professional digital studio in a computer, you will need to look at getting an entire system for that purpose, I would suggest a Mac. If you are looking at smaller scale, you may want a PC though probably still a Mac, just not the top of the line. If you are just doing it as a side thing, getting a pro-sumer level card may be adequate.