AMD Phenom II X4 840T

I'm really hoping i can get some infomation on this, anything leading to figureing this out would be awesome. So, there is a handful of games that has this issue, Tomb raider 2013 is one of them. Everything seems normal, there is no issues with the game running, heck it runs really smooth...

I am trying to upgrade my CPU to the AMD Phenom II x4 840T, and from what I can tell it is compatible with the Dell Optiplex 580... but it shows up as an unknown AMD processor, and only runs at 800mHz. Shows up as 800mHz in CPU-Z too, but is at least able to identify what processor it is...

Hello! Before i get into my title, I wanted to say that i'm a student and with no job (yet) and want to upgrade my CPU on the cheaper side without having to switch out my motherboard or ram. Right now i have a "Phenom II X4 840T " Its unlocked from 4 cores to 6 cores with 3.24ghz overclocked...