I have Ausu P5Q motherboard, with Radeon 6850 and Xonar D2X installed in the very next slot to the GPU (since no other PCI-e slot is available), my Radeon usually runs at high temps (even when idle 60 - 65 C), can I install Xonar D2X in some PCI slot rather than PCI-e.
If you've got a Xonar C2X right next to the 6850 then the chances are the fan on the GPU isn't getting enough airflow, hence the high temps. Larger motherboards will often have an extra PCIe slot far away from the main PCIe slot for entirely that reason. micro-ATX might not have the required spacing, though.
Your options are:
1) Remove the Xonar
2) Swap the Xonar and the Radeon (might put the Radeon in a limited PCIex1 slot instead of PCIex16, which will affect performance - check your motherboard manual)
3) Replace the motherboard
4) Look for some crazy liquid cooled system that doesn't need the fan on the side of the GPU (but might be more than a better sized and slotted mobo).