I get the same exact problem.
I have a MSI NEO 4 Plat. with a Athlon 64 Venice Core (supports sse3), and a Ati X800XL for video.
I have the 10.4.3 8f1099 dmg. Converted it with Ultra ISO to a standard ISO then patched with the Maxxuss DVD-ISO Patcher for 8F1099 v1.1a. Then I burned the iso.
When the Darwin loader came up I used the default kernel and it gave me the grey reboot screen. I then tried all the other kernels and the same problem all over again the grey and black reboot screen.
Am I missing something here?
I ran it in diagnostice and get a no kdp_poll; no debugger device error after a kernel panic. What does that mean?