GO TO
http://www.crucial.com/ and click their system scanner it'll show you what memory you need. As it's likely to be old going by your pc's spec ebay may be the cheapest and best bet.
Desktops are cheap these days and it may be better putting the £40 towards a new one as you'd get a decent one like the spec I posted above for £230 ish. That'd have a much better processor dvd writer large hard drive etc.
Easy to instal just open the case pop it out and stick in the new ones -
http://www.fonerbooks.com/r_ram.htm