Its not compatible with Asus p5945g
You can check these from Asus Website.
There are 4 different versions of this board. Unfortunately, like PeterUK said the CPU E8500 is not compatible with the boards.
There is another Asus Board V2-P5945GC which is compatible with the E8500 (Rev E0).
The boards P1-P5945GC, P2-P5945GC, V2-P5945GC and V3-P5945GC are compatible with the E8500 Rev C0.
Anyway you can use the Asus Link to find compatible motherboard for the CPU E8500.
If you are looking for compatible Intel Board for the CPU E8500, check the link below:
Hope this will help a bit.
All the best,