No need to say "closer" to 640x480; it's exactly 640x480.

Here's a 640x480 image (a direct buffer grab from a devkit oXbox courtesy of Borman at NeoGAF) of Halo 2 in 4:3 mode:

Same image I used for that silly "80p" post earlier in the thread. It's very clear that the rendering resolution and the output resolution were the same for this image, no need to bother doing much pixel counting.

Meanwhile, I found an 853x480 capture of Halo 2 in 16:9 mode:

It's pretty clearly rendered at 480 pixels high; stairsteps along the vertical axis are definitely 1 pixel in height. Horizontal is a trickier issue, but it's still possible to figure out. There's a long edge that intersects the plasma grenade counter; I'm counting ~37 stairsteps over 49 horizontal pixels, giving about 853*37/49 = 644 pixels horizontally.

Accounting for imprecision in my estimation, it's reasonable to assume that the image was rendered at 640x480.