This is a horrible comparison. The Pentium 4 was NEVER offered in a Mac, so the next step up would be a CPU that was offered. Adobe stopped supporting PPC based machines several versions ago.
Yes, unused RAM is wasted RAM. The OS can handle managing your RAM, you don't need to worry about how much is being used.
I know I don't need to worry and in fact it's not useful to worry about it, I just would like to know how it's possible that an empty system uses 8gb of ram, how can it need such an enormous amount of data for nothing more than keep itself running and why since it does not seem specially faster. FYI:
For 64-bit AutoCAD 2014
Windows® 8 and Windows® 8.1 (requires installation of the Model Documentation hotfix) Standard, Enterprise, or Professional edition, Windows 7 Enterprise, Ultimate, Professional, or Home Premium edition (compare Windows versions), or Windows XP Professional (SP2 or later)
Athlon 64 with SSE2 technology, AMD Opteron™ processor with SSE2 technology, Intel® Xeon® processor with Intel EM64T support and SSE2 technology, or
Pentium 4 with Intel EM64T support and SSE2 technology
2 GB RAM (4 GB recommended)
AutoCAD for Mac 2014 System Requirements
Apple® Mac® OS® X v10.9.0 or later (Mavericks), OS X v10.8.0 or later (Mountain Lion) with 64-bit Intel processor
Apple Mac Pro® 4,1 or later(Mac Pro® 5,1 or later recommended); MacBook® Pro 5,1 or later (MacBook Pro 10,1 with Retina Display or later recommended); iMac® 8.1 or later (iMac 11.1 or later recommended); Mac® mini 3.1 or later (Mac mini 4.1 or later recommended); MacBook Air® 2.1 or later; MacBook® 5.1 or later (MacBook 7.1 or later recommended)
3 GB of RAM (4 GB recommended)
Added to the fact that Autocad for mac lacks a lot of funcionalities, how can it be possible that it always needs more hardware even for doing less? Aren't there intel macs with less than 3gb ram or what?
The windows version, running with an athlon 64-pentium 4, implies that it's capable to run on a 10 years old machine.