It's shouldn't mater where you buy the RAM from so long as the stick(s) are
compatable with your computer. All RAM are made to common standards, thus
are interchangable from computer to comptuer regarless of where you buy the
RAM.

As for installation, I think most, if not all RAM comes with installation
instructions.

If it helps, www.crucial.com has a list of compatible RAM with the major
vendors. I added a 256mg Crucial RAM stick to my Compaq notebook, thing
works fine.