After reading your reply, Mike, I realize that I framed my title and question a bit more broadly than I'd intended. What I'm really after is advice from other tarp/bivy users -- in general, do you find that a 5x8 flat tarp plus lightweight bivy provides enough protection for the conditions that I've described above? Or, in similar circumstances, do you find that you're more comfortable with a larger tarp, one with a catenary cut,a tarp with beaks (GG SpinnShelter, etc.), etc? The worst weather that I'm likely to see is a summer thunderstorm, but those can be pretty severe around here.
Also, thanks for pointing out the Golite Poncho/Tarp, Daniel. I was aware that they added one to their line in the last year or two, but I'd never taken a close look. The price is hard to beat, and the design looks nice (plenty of tie-outs, etc.).