I know the price of the X2 3800 is pretty good, but I still think a lower end Conroe (Allendale to be exact) would be better value, especially in the long run. Its performance as seen in the Anadtech benches are amazing:
http://www.anandtech.com/cpuchipsets...oc.aspx?i=2795
Depends on what upgrade you are looking for i guess, extreme budget but already on a 939 system, id go for the X2 3800. Anything older, id dish out for Conroe, a fair few cheap mobos about too!
Kompletts prices look fantastic too:
E6300 1.86GHz £125
E6400 2.13GHz £150
E6600 2.4GHz £220
E6700 2.67GHz £355
All including VAT!