Just wanted to make a post regarding the Overlay in Ventrilo. I'm currently using 3.0.8 x64 with Windows 7 64-bit. I was having issues with it and wanted to make sure that it was enabled so I could learn some of the "newer" members voices. Anyway, after searching on the internet the solution seems to be to ensure that your game resolution is the same as your desktop. Whether this means that you need to increase your in-game resolution or decrease your desktop resolution is your decision. Other than that... it worked just fine for me. = )
If you're wondering how to set a custom resolution in Halo, you can add -vidmode x1,x2,x3 to the properties tab, where x1 is your width, x2 = height, and x3 = refresh rate. I'm not sure if this will allow you to go to a higher resolution than is normally supported via the Halo settings.