It recognizes both sticks of 4gb RAM, only uses 4. Windows 7 64bit

Are you absolutely 100% positive that you are running 64bit?

If so, check this: Go to Start>Run>msconfig

'Boot' tab, Advanced options, make sure Maximum memory is NOT checked off.

Recheck the RAM and please reseat it.. Then make sure to look on your RAM sticks and read the Voltage that it has printed on them. (usually 1.6v)

While your computer is turning on... Check "Memory Remap" option in BIOS and try turning it to 'ON', then make sure your voltages are correct.

Try all of those options and tell me how it works out! Good luck!

I'm confused as to which socket and CPU you have. If you mean i5-2500K, that requires an LGA 1155 socket.

And how "it" says you have 2 sticks in each slot. That obviously wouldn't fit. And what is "it"?
Btw, always use A1 B1 or A2 B2 for dual channel mode. What RAM sticks exactly are you using?

Because of this - "LGA1156 Core 15 and 17 CPU's 2 x 4Gb" - I think that these are older memory modules, designed to be compatible with socket 1156 CPU's. Standard DDR3 voltage is 1.50V, but those older modules often required 1.65V to run properly when using more than 1 module. As MistahYebba suggested, try to figure out which voltage the modules need ans set this as DRAM Voltage in BIOS.

The BIOS will simply read out the SPD on every module to determine what kind of modules are used. Maybe one of the modules is faulty and the SPD can still be read, but the module cannot be used anymore. You can test the modules individually in the same slot to find out.

Try different slots. It could be related to some socket pins that are bent in a wrong way.

If the CPU cooler is mounted too tight, it might cause so much stress on the motherboard that it will disrupt the contact between socket pins and CPU. You can test with a Intel boxed cooler if you have one.