Kev, it will speed it up if the Motherboard supports dual channel. If it doesn't then there will be no speed increase. The m/board bamhm was looking at did support it so I was explaining why 2x512Mb was faster than 1x1Gb for example.
In response to the OP's question:
Being lazy, this is the way I'd do it. Go to
Belarc - System Management For The Internet Age, and grab a free copy of Belarc Advisor. This little gem is free for personal use. Run it, and it makes up a COMPLETE inventory of your machine, and all the hardware and software on it.
Then, if you go to crucial.com, you can search for ram for your exact model of motherboard. Crucial will show you a list of all types of ram that will work on your particular board, or at least all types that they have available. Simple, easy, takes just a few minutes, and there you go. Slick as a politician at a town meeting.