The E-240 suck at overclocking, designed for laptops and are soldered on to the motherboard.
The Sempron 145 is an end of life cpu, sinlge core, but does overclock well, has been known for people being able to unlock a second core.
If $$ tight, go with an AMD Trinity series. A4-5300, although cheapest chip, far superior to the E2-40 or Sempron.