Phenom to phenom ii

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That's not Fm2 but AM2+ MB with AM3 socket processors compatibility added later on.
According to that CPU compatibility list it supports up to 95W processors. Those lists are not complete, only processors actually tested on MBs with newest BIOS are included.
AM3 AMD Phenom™ II X4 x945 HDX945WFK4DGI 3000Mhz 4000Mhz 95W is first on that list just make sure your BIOS is updated to last one.
If that's actually your motherboard then you have an FM2+ socket and that is what you need to focus on when you buy a CPU.

"840 and 945 have the same socket (am3) and the same 95w and on the website there's no 840 on the list..
Will it work?"
Maybe, it's not a native AM3 socket so I'd be careful.

FYI...
I don't think you'll see much of a speed increase...
So, I recommend you save up and upgrade to an Intel iCore series platform.
 
That's not Fm2 but AM2+ MB with AM3 socket processors compatibility added later on.
According to that CPU compatibility list it supports up to 95W processors. Those lists are not complete, only processors actually tested on MBs with newest BIOS are included.
AM3 AMD Phenom™ II X4 x945 HDX945WFK4DGI 3000Mhz 4000Mhz 95W is first on that list just make sure your BIOS is updated to last one.
 
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