i know how to OC the graphics card with ATI Tray Tools / RBE (max. up
to 1.15 V) or Riva Tuner, but can someone tell me how to OC the Vcore
more than 1.15 V cause with 1.15 i only reach 760 (gpu core) and 940
mhz (ram core).

From the KVR800D2N5 RAM data sheet.
Single Power Supply of +1.8V (+/- .1V)
Clock Cycle Time (tCK) CL=5... 2.5ns (min.) / 8ns (max.)
Row Cycle Time (tRC)... 57.5ns (min.)