I've recently had trouble after installing more Ram in my computer. Even though I have a limited knowledge, I thought I'd write a little summary of my findings to help other users. Overall it might be slightly more relevant to second hand ram because they're more likely to be faulty.
The following takes for granted that you got yourself the right speed Ram, that it does fit in the slot, that your computer does boot up and that the extra Ram shows up in the system monitor.
1) Is my new Ram working?
a rough way to check
Since most electronic shops have a 48 hour return policy you want to make sure that your new Ram stick is working. A quick and unreliable way to check that is to start firefox from the terminal (#firefox-3.5), head over to youtube and open loads of HD videos in new tabs. If firefox crashes and the terminal's last line is "Segmentation fault", chances are your ram is faulty. It might be caused by something else, but I found that was a good hint. In doubt, remove the new ram and do it again.
2) Is my new Ram working?
a thorough way to check
Firefox does crash under pressure? Time to turn to Memtest86+. You might have it in your Grub list at boot, if not pop in a live CD and it will be there. You will want to use a recent live CD because older ones have older versions of Memtest and those might not use the full potential of your processor if it's a recent one, thereby multiplying the time it takes to check the Ram. Look in the top left corner. I have a quad core processor but older Memtest would recognise it as a Pentium III and the check was going very slowly. If needed, get the latest version of Memtest86+ http://www.memtest.org/#downiso burn it to a cd (you might have to remove the problematic new ram to be able to do that) and boot from it.
Memtest goes through multiple passes and the funny thing is that errors might be missed on the first pass. Once you reach 100% on the pass line at the top and the screen tells you something along the lines of "No errors found, press Esc to reboot" well, don't. Just hang in there. In my case it took another hour into the second pass for the errors to show up, but it might take you 4 passes and 3 hours, who knows...
3.1) After multiple passes, Memtest86+ didn't find any errors.
Okay so it's not the Ram. Great, what then? Maybe you knocked something out of place whilst putting the Ram in? Time to open your box and check everything: cables, processor fan, the whole lot. If you still experience random crashes, be honest, you already had those before putting in the new Ram didn't you? You cheeky sod!
3.2) After multiple passes, Memtest86+ did find errors.
Ah, so it WAS the Ram. You can hopefully bring it back to where you bought it and exchange it for another stick or get your money back.
4) "Too late, the shop won't take it back" or "Someone gave it to me": should I just throw it away?
That is awkward. You have loads of new memory but can't use any of it because of a few errors. Well, don't give up yet. Why throw away 1GB or more when only a few MB are faulty? Maybe you don't have to. There is a couple of ways you can still use that faulty Ram without having programs randomly crashing. First be aware that some people say that it's not a great thing to do and that a bad stick of ram will just keep and getting worse, but if you don't have the cash to buy more, it's worth considering.
-The first way is called BadRAM and you will need to recompile the kernel, something I wouldn't do because I don't know enough about Linux and it sounds well scary, but feel free to give it go. Here is the link: http://rick.vanrein.org/linux/badram/
-The other way is to use Memmap. Just like BadRAM, it stops your system from using the erroneous part of the ram. Here is a link to a How-To http://gquigs.blogspot.com/2009/01/bad-memory-howto.html. If your using Grub2 (Helena) pay close attention to the last post on that page.
Here you go. Hope this might help someone.
PS: If you're an admin and find this post vague, superficial or downright dangerous, feel free to remove it