PCI interfacing is a non trivial exercise. The first issue is the hardware, whilst there are chips to do the hard work, it's not a simple or cheap exercise. Then you have the software - writing a device driver for windows (or linux) is again, not a simple exercise.

May I suggest a much simpler method is the use the rs232 port or if you require higher speed, try a USB/serial chip like the FTDI device. Much simpler to interface to a 8051 (or other micros).

As for interfacing to the IDE bus, it too has its problems as most IDE buses from PCs run way to fast for an 8051 to cope with without having hardware support.

In the case where you want the 8051 to be the master to talk to an IDE device - there's plenty of MP3 player designs that do this on the web along with example code.