Thanks everyone for your input.
I went ahead and ordered a pair of 4GB sticks - 2x PSD24G8002 in fact. I'll report back my results when they get in later this week.
The price was a bit expensive (compared to DDR3), but I have another system these will work in if they don't work in the Optiplex 755 USFF. That system currently has 2x2GB that I probably can put in the 755 USFF, so I'm probably still ahead regardless of the outcome.