I'm a big one to not "jinx" things, especially since we'd been trying so long, and had a loss after we got the A-OK from our doctors... But, I admit, this time I started picking things up around 9 weeks or so. Maybe sooner

I found a really nice barely used breast pump on CL for a great deal, had to get it! Diaper covers on Freecycle- snatched them up! We even ordered our stroller already, since it was on super duper sale.
We've like
BabyWang suggests, we'll also register for and have purchased mostly gender neutral colors/styles, even for m beloved CD stash. For one, the U/S is not guaranteed 100% accurate, and we'd also like more than one, so being able to re-use things will be a nice bonus.
I'd say if you see something that's a good deal, go ahead and get it if you're really excited about it! Nothing like putting it out there to the universe that you know things will be OK!
ETA: re: maternity clothes- I got 2 pairs of jeans around 12 weeks since my regular jeans were really uncomfortable. I think posting on FC now is a good idea, since you may have to wait a few weeks, and post again to get what you really want. I can't stress enough- ask for pics of the FC stuff before you arrange pick up! there can be some real winners out there- and some not so much. (I got lucky!)
ETA2: usually, if you're gaining an "average" amount of weight, then you go with your re-pregnancy size. If you seem like your gaining a little more, or you see that you're gaining more in one particular area, then ask for one size up. (I got some XL shirts that are HUGE on me, and usually XL fits well. these are so big, i doubt they will even fit in 5 months!)