if you want it to run very fast and nice all arround the world, you need a server, i am an IT tech IRl and have many servers in my house, one hosting Plex
TBH... even a dual core ATOM running WinXP can run PLEX(Prior to v1.0) LOL.
The only time you need anything beefy with PLEX is when you are trying to transcode 720/1080 content on the fly, especially to multiple users, but PLEX has an "Optimize" feature now that will encode what you select before hand which makes life even easier. Hell, if you have your media setup right, you don't need much of anything for PLEX to do it's job.
Only other thing that PLEX might like is an SSD for thumbnail cache, and that's it.
So, take the, "i haz survorz and am a rl techie" and shaddup. Virtually everyone here works in, or has worked in IT in some form, or another and most know what they are talking about.
EDIT: I also have a server hosting plex... Wait, it's not hosting PLEX, it's hosting ESXi, which in turn is hosting Win7, which is a P2V image, along with Server 2k12R2, a webserver, two instances of PFSense, a copy of OS X 10.11. Doesn't take much to run anything these days, hardware has for the most part, severely outpaced software demands.