Motherboard: M2N68-LA (Narra2)
Processor: AMD Athlon 64 X2 4000+ 2.1ghz
RAM: 2 Gbs of DDR2
Video Card: XFX 9800 GT
Power Supply: Ultra 650 watt
Hard Drive: Samsung 289 Gb
According to my motherboard specifications, processors supported are up to Athlon 64 X2 5600+ under 89 watts, but I want to know is can any processor under 89 watts and can fit in an AM2 socket work here? Say a AMD Athlon II X2 240 Dual Core Processor, I know AM3 processors fit in the AM2 socket, and this particular cpu is under 89 watts.
You've got an old board in an HP, although an Athlon II X2 would fit in your socket, it won't work. The motherboards BIOS won't recognize the processor, and likely it won't work. There is also a possibility it will work, but remain at 800mhz clock speed(some Phenom II X4 chips do this without a BIOS update). OEM manufacturers rarely update their motherboards BIOS to support new processors, they would rather you purchase a new computer from them.