Laptop RAM - explanation please !

[#1]

I want to buy more RAM for my HP Pavilion ZV 6115 Notebook.  I have been looking on Ebay but am mystified by the variety of RAm available.  Can anyone explain ?
HP have advised that I need 1.0GB, 333MHz, PC2700 DDR-SDRAM Dual In-Line.
I have seen DDR2, and SODIMM, and 184 pin and 200 pin.  I have also seen a variety of MHz, some lower, some higher.  I am rather mystified by it all !
Equally, can anyone recommend a source of cheap RAM, even second hand ?  I got a second hand RAM module for my desktop computer a year ago, and it is great but cost a few pennies ! Maybe I was lucky.

[#2]

Your best bet is to go to http://www.crucial.com and put the laptop model in there - they are cheap for memory should you wish, and if your laptop isn't listed (which is unlikely) you can download a tool which will advise exactly what ram is currently installed. This will give you a lot more help when searching for memory second hand.
Hope that helps,

[#3]

I'd definately agree with this approach.
The downloaded tool is by far the easiest option, although entering the make/model on the site does return results in most cases too.

[#4]

Also notebook memory is not the same type and size as PC memory due to its limited size, so be careful you get the correct memory stick format for your notebook.

[#5]

Thanks for the link.  That was extremely helpful and, not only do I know the architecture of the machine, I now understand a lot more about memory modules from the information on the site.  However, I still have two questions :
1.  There is evidently a difference between high density and low density memory.  High density is much cheaper than low density, for reasons which I understand.  It is said that in some laptops, the system will only "see" half the installed memory int he case of high density modules, and they may run a lot slower.  Is there any truth in this before I run out and buy some ?  I don't want to pay for low density if I do not really need to.
2.  HP have told me that the first memory module slot is underneath the keyboard and needs a technician to access it.  The second slot is accessible from underneath the notebook itself and is easy.  I currently have two 256 MB modules.  If I install a 1 GB module, can it be installed in the more accessible slot, or must I put it in slot 1 underneath the keyboard, which will involve takng the whole case apart ?  I hope it does not matter where I put it.
Advice most appreciated.

[#6]

Hi there,
To address your queries:
1. Don't buy high density ram. It's only compatible with about 10% (iirc) of computers though it should tell you on the laptop manufacturer's website if your machine does support high density ram. Unfortunately I doubt that this is the case.
2. Either slot should be fine for this. I'd agree in that you really shouldn't take the laptop apart (especially if it's in its guarantee period!) to install mre ram, and from experience with my old thinkpad, they're a little more intricate than your average desktop once you start taking bits out (I installed a wifi card, including putting the little aerial wires up behind the screen - you would not believe how many pieces have to come out before you can get the screen off) but it should work perfectly in the bottom slot.
Hope that helps,