You're components are very good, but also very revealing. Because of this, you may want to consider entering at a higher level than an absolute beginner. Otherwise, you run the risk hearing all the shortcomings in your PC front end that your amp, preamp and speakers are capable of revealing.
As far as Mac or PC, people seem to be getting very good results with both, providing you set it up properly. I think the safest way to go, in your case, would be to go from a digital out on your PC to your current DAC. It won't cost anything and you can use it for a reference. Even if you hold off on buying more expensive gear, your system is definitely revealing enough where you can experiment with different file formats and media players. You can get started ripping with a free program called EAC. (Just make sure you download one of the setup guides because EAC has so many features, you'll definitely need the help for best results.) Once you get is set up properly, its easy to use.
Once you have a chance to play around with all the different aspects of computer audio, you'll be in a much better position to pick components that best suit you. Its not plug and play like a CD player. There is a learning curve so it may be a bad idea to start buying expensive items right away.