personally, unless you can get a good price on the cpu, i wouldnt. id save up for a full system change now. DDR is getting very expensive vs DDR2, 939's are starting to suffer badly from EOL price inflation and there will be no PCIE 2.0 boards for 939 ever to my knowledge, which could prove an issue in the not to distant future.
the upgrade to an x2 (solong as you get the same or better core speed, meanign a 4600, 4800, FX60, opty 180 or 185) WILL provide a nice boost, and some decent future profing, but its the last straw for s939. after that theres nothing more you can do for cpu speed.
so, if you can get a cpu cheap then go for it. but if your thinking of spending £150 or more on it.. id seriously consider making do for a few months and getting either a c2d or an am2+ based system.