6 GB installed,4 gb usable and bios sees 4 gb

What motherboard do you use? Give me specific hardware specs also. Windows7 64-Bit should have no problem seeing all 6GB.

I have Gigabyte P55-UD5 rev 1.1.I will try memtest and check if one of those rams is not working,is that possible?DXDIAG and CpuZ sees 6 gb

Oh, ofcourse. Your motherboard is based on the Intel 1156 Platform using the P55 chipset which supports Dual Channel Memory not Triple Channel. Your CPU is either an i5 or an i7 860 right?...well it needs dual channel ram. For 4GB you must get 2x2 and for 6GB it must be 2x3 not 3x2. Triple channe isl for the 1366 platform w/ x58 chipsets not these.

oh,thank you very much.Then I will get one more 2 gb ram..And yeah,I use i7 860.One more thing,my FSB RAM ratio shows 2:12 in CpuZ and I want to make it 1:1,how can I do it?

FSB RAM ratio?...well not sure about that. But first install some dual channel memory and see it works then we'll deal with other insignificant problems like ratio. Hope that helps  

I've found that stores dont sell single version of my ram,8 gb is too much for me already.I decided getting 2x2 GB Corsair dominator gt cl8 2000 mhz,little more expensive but its ok and I'll change this tomorrow   Most people say 1:1 ratio provides the best performance but I don't know how to change it and 2:12 sounds really stupid,even default ratio must be near 1 as far as I've read.

You don't need to buy a new ram kit. Just take one module out and you will have a dual channel kit. You can probably buy one more matching stick to the one you have online.
OCZ uses the same memory in both dual and triple channel kits for example.

I paid for 3x2 GB and now third ram is totally useless for me.I prefer 2x2 GB with better CL,better FSB and thats why I'm gonna exchange them.One more ram is not wisely I think,because 8 GB is waste.

My resource monitor says 2053 mb is hardware reserved,if third slot is not enabled why does it say this?

Well,I've fixed problem now.Its not related with processor or motherboard,I've fixed it by changing DIMM slots and now bios,task manager,cpuz,system properties show the same amount:6144 MB without annoying "USABLE" thing and it works as dual channel also   Thanks for help

which dimm slot did you use
ı already have the same problem

Click the Start orb, and type in msconfig.

Right click the msconfig app to run it as Administrator. That brings up the configuration utility.

Click the Boot tab, and then click the Advanced Options button.

In that window, there is a setting for Maximum Memory - Make sure the checkbox for it is clear (not checked), then reboot your computer

I have 64-bit Windows 7, had initially installed 2 x 2 GB RAM.
I bought another 2 x 2 GB (exact same RAM) and installed it; got "8 GB, 4 GB usable."
After following various threads I was inspired to check whether the DIMMs were seated; one of them wasn't. I pushed it in, and voila -- problem solved.

You can't. The memory has to run as a ratio of the base clock speed. As long as your RAM is running at the proper speed and timings, don't worry about the ratio.