Multiple problems with new PC

MixedMartyr

Reputable
Jan 20, 2016
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Recently built a new PC with both new and used parts.
AMD Phenom II X6 1035T @ 2.6
Asus M4A89GTD Pro USB3 motherboard
8GB(4x2GB) RAM from an HP
GTX 550 Ti

First problem is that only two RAM slots on the motherboard work, thus I have to use only 4 GB of RAM as opposed to 8. Is there any way to fix this without buying new RAM or just buying a new motherboard?

Second is the processor. It's only showing 5 cores are active, and when I go into the bios, it only shows cores 2 through 6.

Third problem is gaming performance. I've been playing Rocket League and Overwatch for a few months with my 550 at a solid 60 fps. Now that I've upgraded my processor but kept the same card, running these games with the same settings drops to around 30 fps and is really choppy. Could this be from the missing core, less ram, or something else?

Let me know if there's any more information that I need to provide.
Thanks