Weird solution to Halo performance

This game has been running like crap on my system. It hasn't even been
playable at 800x600, or 1024x768. I've heard about issues with AA so i
always turn it off. My system is 1.2 athlon, 512DDR, 9700pro, 4x agp.
Well i thought i would do something really stupid, out of boredom. I bumped
it up to 1600x1200 to see how low i could go with my framerate. I could let
it sit, and just watch the screen at 16x12 and dream about what it would be
like to see it move smoothly at such a nice res. Low and behold, it runs a
100% better at 1600x1200! Why??? It's actually playable now, and if i had a
faster cpu, i bet it would run even smoother all the time. It's really
awesome to finally see this game in hi res, but i don't understand why it
runs so much better at 1600x1200 than 800x600. The only thing i can think of
is that by raising the res, it's telling the card to take more of the load
rather than the cpu? With the exclusion of AA and AF, this is true of my
other games too. I get better framerates at 1600x1200 than i do at 1024x768.