Is this safe?

well, i think u need to be more specific, what kinda mother board u have.
whats the specs for the pny? what the specs for the apacer...
otherwise how would anyone know?

Hi.

Well, in your case with 1GB you won't see much difference, try to get another 2GB.

Now, What specifications have you rig? Mobo, CPU, GPU, RAM. And the most important, What is the main use of that rig? Games, surf the web, video edition...

First : Games
Second : PNY DDR2 800 2gb, i plan adding some more ram but cant get any locally(other country me lives) and i plan on adding DDR2 800 APACER 2gb
Third:I am asking one question and expect one answer, is it possible to use different branded RAM together, Yes or No and explanation only.

Yeah, is possible use difference brands. Just make you sure that your new RAM have the same voltage and CL of your old RAM.

They will have to run at the slowest timeings between the two if the pny set runs at lets say 4-4-4-15 and the acer runs at 6-6-6-18 you will have to run them at the 6-6-6-18 timings which will be slower and will not be worth upgrade.

Note: The timings i listed were just an example i have no clue what your exact timings are so take that into consideration.