Frustrum-Triangle visibility/intersection test?

My landscape is determining which quads to draw using point tests at the corners of bounding boxes. It works ok, but it is possible that when very close to the camera, none of the points in technically in the view frustrum, so the polys never get drawn... To do this was simply a matter of transforming the points using the world/view/camera transformation matrices in D3D8. Now, it seems I need to actually compute a frustrum-triangle intersection test.... Or is there an easier way? Perhaps I could transform the 3 points of a triangle into screen space, and test if that 2D triangle intersects with my screen viewport (in pixel coordinates)? What is the best way of doing this? Thanks! Mal.