Have you guys tried using EFI strings if you're on a Chameleon (or similar) install? Most NVIDIA cards (including 8400GS) are supported this way and tend to give pretty good results.
I'm on Snow Leopard now with a 512MB 8400GS and I couldn't get any of the NVInject-style kexts to work well enough, so I went with EFI strings but I was still having some graphics problems like getting stuck at a blue screen on resolution change, very delayed (I mean it took hours) color inversion when using Ctrl+Opt+Cmd+8, full screen games weren't working and there was just general choppiness when using either DVI or VGA. The problems were there in Leopard as well but I never found a fix.
Today, just for fun I tried setting up 2 monitors as an extended desktop using both VGA + DVI ports at the same time and boom my problems magically went away. My resolution now changes perfectly fine, color gets inverted like it should and animations are a whole lot smoother.
It also works if you just plug a spare cable in to the spare VGA/DVI port that you're not using – it doesn't even have to be attached to a monitor.
Thanks for this great tip. And indeed it switches resolutions fine now. Thank you, thank you, thank you!
In fact I investigated this a bit further. For single monitor setups you HAVE to connect your monitor to the DVI-port and put a spare cable into the analog port. Otherwise this trick wont work.
Using the analog port and leave a DVI-analog adaptor also doesn't work. You really need to plug your monitor on the DVI-port (either through an DVI-adaptor ot DVI-analog cable) and leave the spare cable attached to the analog port.
For your info this is my system.
Pentium 4 630 (3GHz Hyperthreading)
ECS 915-P mainboard
Asus GF 8400 GS 512MB DDR2 silent (the one with the big silver heatsink)
2GB DDR1 Ram
OSX 10.5.2 (Kallyway DVD using the included NVinject 0.2.0)
Thanks again to LennyX.