AM3 cpu wont work on AM2 mobo

Looking at the CPU support list for that board I see there are 2 versions of the Athlon II X4 640, the difference is in the stepping C3 vs E0. Since E0 is the newer stepping and the one not supported, it's likely you have one of these.

Time to flash BIOS !!!!...... with a strobe light.

Amm sorry i didnt understand your question - what i must do now ? sorry im from Latvia lul

You mean update ? i did update it already

What loneninja is saying, is that there are 2 versions of your CPU. One is supported, the other is not. So, most likely your problem won't have a solution.

What i was saying, just a silly anekdote. I'm Portuguese btw.

BUT where do i check is my cpu C3 or E0 ? i cant fnd it on da box

The CPU should have a serial number, something like this AD3500OJGXBOX or this AMDHDT45TWFGRBOX. Just google it, it will says witch version ti is. Or maybe is ON the CPU it self.

Amm i have C3 model because my box says ADX640WFGMBOX please help me i did wait for multi core processor for years im so desperate !

Ok, then. Reset the BIOS and start removing everything fromt he board. Leave only CPU (with heatsick ofc) , MB , 1 Dimm only and the PSU. And GPU or integrated if you have. See if it posts.

As part of your BIOS update, did you clear the CMOS and reset to system defaults?

Yep i did pull it out wait 30 secs and put it back

Well i did everything like you said and it didnt work same scenario ....