http://www.crucial.com/uk/index.aspx 
My 2nd PC, an old 2004 'self spec' off E-bay is running a little slow so thought I'd buy some RAM, this is a great site to check and upgrade yer RAM - prices on the money too, have been told the older DDR RAM is actually going up in value as more people try to upgrade as opposed to buying new so if that sounds like you then try this site, just download the free scanner and it will tell you how much RAM you have and what type of Mo'board, CPU etc you are running and will offer you upgrade options which are dead competitive. IIRC Kingston are very slightly cheaper for RAM but both are the best in the same way that Intel & AMD have the best CPU's.
Excellent site for RAM
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