Valid GL_PROJECTION switch?

My goal is to have a 3D map with a HUD system. I was thinking of doing this (but is this valid?) I wanted to draw my scene with
gluPerspective(45.0f, fAspect, 0.1, 1000);
And then switch projections to
glOrtho(0.0, 800.0, 0.0, 600.0, 1.0, -1.0);
Turning off my depth tests and such, and draw my HUD. Is this switch valid on the fly? Or is there a better way in which to do this?

Switching projection matrices during the frame render is perfectly valid, and probably the best way to draw a 2D layer over the 3D scene.