music server setup questions

I am going to get a computer and rip all my CDs lossless and use the computer as the transport. I've no idea how to approach this. My dac is old - Theta ProBasic IIIA. I can get a computer with a digital output that plugs into that, or I could get a USB dac and replace my old Theta. Any thoughts on that? Also, any reason to go PC vs mac, and what software will I need/use (this computer will mainly be for music but I will also do basic web browsing and word processing, nothing fancy)? Haven't been on forums in many years. Rest of system is Simaudio P5 pre, dreadnaught II amp, snell xa90ps speakers. Thanks in advance. -Dave