i dont see the problem here,
you said that it worked fine on the single stick of 512.
i suggest buying your ram all at the same time from now on, and that way you will be sure they match.
and as for the hard disk activity light, its as good as forgotten.
but next time dont tell us stuff that doesnt matter if it doesnt matter. if it isnt on the subject, then dont mention it. if it is on the subject, dont get mad when you find out how much you dont know.
we only know what you tell us, so if you babble like a fool about a light you dont understand, then we arent sure where to direct our attention.
but about the memory.
i would suggest you go to www.dogpile.com
and search for aida32
it has a memory benchmarking tool that can tell you if your memory is in fact slower when you add it all.
test it with the smaller stick, and make sure to run the test 4 or 5 times to get a good average.
then, test it with the larger stick, and make sure to do it 4 or 5 times.
then tell us what it told you.
till then i have no idea what to say.