The ever-present anti-virus apps and anti-spyware apps have taken my work Windows box that, 18 months ago, ran just fine and turned it into a doorstop at times. My CPU is constantly being hit and memory being utilized by these apps. Sometimes my hard drive gets hammered bythem, too. It then becomes time ot walk away & drink a Coke. This is a company machine & they set the policy WRT all those services. Yeah, I could fiddle with their mandatory settings (AV scan @11AM, AS scan @1PM), but I got to work with them on other projects.
Administering a Windows box has gotten to be more of a pain because of the AV, AS, firewall, OS patch issues. Expect to spend much more time administering your new Win box than your Mac.
Another processor (& a commensurate boost in RAM) would be welcome, indeed, to carry the OS, AV & AS overhead.
IOW, to run Windows OS on white-box PC hardware, you will need more/better hardware to get performance similar to Mac OSX or any of the contemporary linux distros.
If you go for PC hardware, make sure your mobo supports PCI Express. Most dual-proc mobos are intended for server work & lag behind the cutting edge WRT some of the details (AGP vs PCX for video, in my buddy's newest box).
Good luck with your decision & new box, whatever arch & OS it may be.