I can't seem to find what the max amount of ram is supported on the Acer Aspire one D255. It came with 1gb, and I see that most netbooks support up to 2gb.
"". The D255 will support 2GB of RAM (there’s a single slot inside) but it’s not especially easy to open up the casing. Happily you’ll only ever do it once.""
While the Slashgear site certainly seems to suggest that a single RAM slot leads to no more than 2GB of memory, DDR3 RAM is available up to 4GB on a single module and could in the future go as high as 16GB on a single module.
Therefore I'd say the question is still open. Perhaps Slashgear is wrong or just sloppily worded. Perhaps there is a 2GB limit but if so it's not because a single DDR3 slot can't have more memory plugged into it.
It turns out the Intel Atom N550 is limited to 2GB of RAM, so the processor is the limiting factor rather than the number of RAM slots.
your acer aspire d255 is most likely a 32bit system. If its windows 7 starter it will only allow you to have 2gb of physical memory. You can change that by changing settings in your boot menu though.