Update: Found Solution for this!

There is a small and handy Application called About This Mac found at Insanelymac

The Function of this Application is pretty simple, it just adds your current CPU Information as the default-value for unknown CPUs

The command to show your current (real) CPU-Information is
sysctl -n machdep.cpu.brand_string

The file where the Systems unknown CPU-Information is stored is:
/System/Library/CoreServices/Resources/YourLanguage.lproj/AppleSystemInfo.strings

The Value to change is named UnknownCPUKind, "About This Mac" does this automaticly, i just tell this for people who want to do this manually.

After you installed About This Mac (and of course rebooted your System) and your CPU still isn't shown as expected, i found a workaround for this, but it is pretty dirty.

Just put CPUInjector.kext from here in your /Extra/Extensions/ or /S/L/E Folder.
After that edit the Info.plist file in CPUInjector.kext

Just add any invalid value for cpu-type (so that your CPU will be recognized as unknown CPU on the next boot)

Example from my CPUInjector.kext/Contents/Info.plist:

<key>cpu-type</key>
<data>
AMD=
</data>


Now my CPU is shown as AMD Athlon(tm) 64 X2 Dual Core Processor 4200+ in about this Mac. The only bad thing is that it doesn't show up in System Profiler anymore.

I also tried adding invalid SMcputype in /Extra/smbios.plist, but this did not work for me, and my CPU still got shown as Core 2 Duo.

Maybe there is a more simple way for doing this, but since i had no idea where to start this works pretty good for me.