PC only uses 3,6GB RAM while 5GB installed on x64 system

Well i know there are many threads about this issue but im not gonna read them all, so far ive not found the someone with my exact type of problem.

Im gonna try to explain it as best as i can.

you know what is written in the tilte so here we go.

first of all on my PC ive had always windows 7 Ultimate x64.

the problem started when i lost my raid 0 setting because ive had overclocked my CPU too much and my pc would't boot anymore. so i had to reset the BIOS and with that i lost Raid 0 and ofc my windows installation. I thought my CPU was broken as well because from then my PC was very unstable and constandly freezing.
so i bought a new CPU an older one a Intel Pentium D as you know less fast then the newer Dual cores.

Ive installed the Intel Pentium D and i put a new Graphics card in too because that was causing the Freezes.
The Pentium D could use the full 5GB on Windows 7 Ultimate x64 but was slower. I got from a Friend a Intel core to Duo at 3.6 ghz, so i installed that CPU. But when i got in to windows i saw only 3.6 GB was usable of my RAM. Never touched my RAM setting.

I find it strange that a older CPU can use 5GB of ram and a newer CPU only can use the 3,6 GB

Well i thought im gonna install my old Intel core to DUO that i thought i had destroyed by a faulthy overclock. But it works just fine, but also i can still not use the full 5GB of Ram but the strange part is that this CPU is the one that first accepted 5GB and now only 3,6 GB.

So what could cause this? it this windows Related?

sure that the rest of the ram isn't used by a ram-less onboard graphics card?

My motherboard has no onboard graphics card.

Open Resource Monitor, switch to memory tab and post a screenshot please.

Most likely a misconfiguration of RAM. Try changing the slots, and changing the RAM to auto timings.

also your ram was misconfigured from the start. you can't have 5 gigs in a dual channel setup.
use 2 identical sticks, not three.

when i build my pc, i could use the whole 5GB and its not in dual channel, its in flexmode.