I've encountered a problem with Halo when I upgraded from a 9800 Pro to a 6800 GT. I lost the cloaking effects in halo!! I believe this is because the game detects Nvidia hardware and assumes it is an FX and defaults to lower precision and removes to "taxing" effects. How do I get those effects back? I've heard you have to fool the game into thinking its running on ATI hardware by changing the card ID, but how do I do that?
"<i><font color=purple>-Use20 Forces the game to run as a Shader 2.0 card. This shows all graphical effects in the best possible visual quality. You will need a recent graphics card with full PS2.0 support such as the Radeon 9500 series or higher, or the FX 5600 series or higher. PS2.0 gives the biggest performance hit but runs Halo exactly as the developers intended it to be seen.</font color=purple></i>"
(love the mention of at least an FX5600, yeah SURE!)
My personal favourite Tweak guide for Halo where the above came from;