A simple rule I always tell people who ask me about Rockit:
For playing audio only, if the computer can run the OS, it can run Rockit. Rockit uses little resources for actually playing music. The CPU, memory and hardware stuff, is more related to HOW FAST you want it to run, and how robust you want your searching, sorting, display, etc.
I had one gentleman argue with me recently, because he said Rockit cannot handle unlimited tracks in its library. He had over 200,000, and was running it on a lappy, and said it was slow, and that meant it was not unlimited...
So, you should size your system according to the speed you want out of it. Putting 200,000 tracks on a slow external drive, using a reasonably slow lappy, will not produce the best results (although it will work)...
If you will be using a large library, you will want a faster and more powerful machine all around -- if you have a reasonably sized library, just about anything will do, and again, it will only affect robustness of a few things, not the music playback itself.