Should I turn it on or install memory first?

I know turning it on first and then installing it works, and I think the other way works too. I really don't think it matters though. If you go to system profiler, (apple menu, about this mac, more info) it will tell you the slots that are filled.

 

Sometimes it's fun to wait. Turn it on and work with it a few days, then install the ram and see if you notice a difference. It makes the purchase feel justified and warranted. If you install it before actually using the machine, you may never know how much it is helping.

As the Audio Guru Gerry Lemay said [concerning all things audiophile] "if you have to sit around, drink wine and eat cheese to notice a difference; it's not worth it".

 

If you install the memory first and there is a problem, you know if the problem is caused by the new ram or was pre-existing. Starting up first can help diagnose problems.