Before spending any money, you need to do more research (unless you're very rich of course).
I assume your "Anthiom" is a mangled name for an AMD chip...and many of these are as good or better than Pentium and may well run your Cubase upgrade. They have names like "Phenom" or "Opteron" or older ones like "Athlon" (which, when it came out, was faster than that generation of Pentiums).
I'm not saying you don't need a new computer, just that you need a lot more info before you rush out and spend 3 grand on a Mac you may not need.
...and, as others have said, with Windows you'll probably get the same computer power for about half the Apple price. Just about every Windows "problem" people talk about is, in fact, because the load up their machines with tons of downloads and rubbish. If you keep your recording computer as clean as you say, you'll get little or no advantage for the extra spend to get a Mac.