Hey there, so I bought a ddr2 800mhz PC2-6400 8gb ram sticks (4 x 2gb)

I put it in the computer and it runs and it’s visible in the bios , but the only issue is it’s running at 667mhz , I contacted the seller and they are saying my computer doesn’t support the 800mhz that’s why it’s not running in 800mhz but I remember seeing threads saying the optiplex 755 runs PC2-6400  800mhz . Is there and option I was meant to see the in bios to make it run at 800mhz .