From Offtek you can get 4×FTG128MLQ64V8J:
http://www.offtek.co.uk/ddr2.php?subcat=70&subcat1=44&subgroup=1&memtype=DDR2-PC6400 (4×£11.18=£44.72)
See this thread: http://en.community.dell.com/support-forums/desktop/f/3514/p/19500496/20344742.aspx#20344742 for more details.