Best advice.
Go to Best Buy, get a Dell (ignore those bagging on it - it's trendy to rip Dell now). It saves you from having to know how to put it together and maintain it. And because it's Best Buy, if something breaks, or you have a question, you can just run over with the case...Good luck with that buying online and/or building yourself.
I have a high-end Dell from Best Buy that runs perfect. I play WoW on it all the time. It was only $1500 and I added RAM. I also have a custom liquid cooled PC -- runs the same...identical ('cept the liquid cooled one gurgles/burps at times, and is loud).
You can also do the Apple thing if you're ok paying the "coolness" premium -- it plays WoW just fine too - I have a couple Mac WoW friends.
If someone wants to play online gaming, definitely spend more and go all out. You will need it.
If someone just wants a steady work machine that'll let them do their thing, a $400 refurbished box is a good way to save money. I don't think I'd be any more efficient on a $3,000 computer than I am on a $300 box.
Further, I don't buy from Best buy - I get it online, built by some company in Arizona who builds boxes for companies. I guess it's peace of mind for me... if I have 1 supercomputer, and it crashes, I'm in trouble - it's my whole business. Rather, I like backing up everything every night. When a computer dies, I just plug in the next one, swap out backup USB wire, and start working.