Can anyone help me with calculating a Frustrum Bounding Volume, or obtaining the four corner vertex's. Im using directx. Thanks

Aren't there 8 corner vertices?

But anyway assuming you do mean 8 and not 4 it's done as follows (well, I'm an OpenGL man but here's how I think it works in D3D): take your 3 matrices (world, view and projection), multiply them together in order (i.e. work out the overall transformation matrix), and then invert it. Then transform 8 vertices through that which are all the combinations of (-1/+1,-1/+1,-1/+1). This will give you the 8 points in world space. Which is which depends on whether you're using a left- or right-handed coordinate system (I seem to remember most D3D apps use left-handed in defiance of hundreds of years of science and maths, but I could well be wrong), and the matrix inversion will be different to OpenGL because the matrices are stored differently. If you want the viewer position, transform the vector (-1, 0, 0, 0) through that matrix.

Finding the clipping planes didnt help me much becuase i want to check the frustrum volume against my octree & the clipping planes tell me if a point isnt in the frustrum where-as I want to check to see if the frustrum is inside a quad.

I coded up to find the eight points of the frustrum and created a virtual camera(world view projection) so that I could render it all on the screen. It draws a frustrum shaped pyramid on the screen :) - but changing the world matrix moves it in the opposite direction - obviously becuase of the invert - removing the invert and i dont get a pyramid frustrum. Inverting the world matrix before the multiplication works. Is this a correct behaviour?

Added: Somthing is deffinatley wrong, changing the farPlane increases the size of the frustrum but also moves its posiition. Am I multiplying the matrix's in the correct order?

Heres my code:

Matrix p = Matrix.PerspectiveFovLH((float)Math.PI / 4, 1, 1.0f, 2.0f);
Matrix v = Matrix.LookAtLH(new Vector3(0, 0, 0), new Vector3(0, 0, 2), new Vector3(0, 1, 0));
Matrix m = Matrix.Translation(-3, -1, -3);
Matrix M = m * v * p;
M.Invert();

fv[0] = new Vector3(-1, -1, -1);
fv[1] = new Vector3(1, -1, -1);
fv[2] = new Vector3(1, 1, -1);
fv[3] = new Vector3(-1, 1, -1);
fv[4] = new Vector3(-1, -1, 1);
fv[5] = new Vector3(1, -1, 1);
fv[6] = new Vector3(1, 1, 1);
fv[7] = new Vector3(-1, 1, 1);

for (int i = 0; i < 8; i++)
{
fv.TransformCoordinate(M);
}

Thanks