Okay, so this is all a matter of opinion. I bought all my stuff now (not ALL just the stuff I know I will need before baby arrives and the rest I'll get at my shower after the baby is born). We set up the nursery, painted, put the bedding up, etc. I will say that it has never once made me sad (okay once when I had to "dust" it I was sad....) but other than that it makes it feel REAL. It makes me feel like this is not like infertility and won't end up going nowhere. This WILL happen. That's what I get when I walk in there. I have gender neutral stuff and found REALLY cute things, but I will only buy something if I REALLLLY love it so everything in there is special to me.
Some people it can make them sad, but I say to do it BEFORE YOU ARE MATCHED, that way, your nursery is for YOUR baby, not that specific one...KWIM? Everyone who has done this that I know of loves it and am GLAD that they did it....
But again, I know many who didn't and that is fine too!! Just whatever your personal limits and fears are...
Good luck!