It looks like you're looking into investing in prestine vocals. AND to get just that is $$$$. A mac or a pc is not going to improve your mic signal. Getting a fast computer is the least of your worries now. FYI, Ram is probably the most important your going to want to look into, cause thats goin to make your DAW run smoother when running multiple plugins. Trust me, any motherboard in the past 2 years is perfect and most of them are duo core capable, which is enough to record anything.
What you want to invest money into is your Mic-Pre. Spendin 1k alone on a mic-pre is solid. BUT in my opinion the True P-Solo is under a rack (1k) and it does the job fine. Soundcards these days are rip offs, cause they add shit you dont need. Just look for one that suits your inputs, so you can plug in your mic, keyboards, mpc etc. As for a mic, a shure SM58 or an SM7b are all good for Rap.
BUT ALSO, your mic signal is just as good as the environment you record in. Good Luck. Hope this helps.