I'm having the exact same problem on an Asus A7N266-C based system. (I posted about it 2 days ago.) I have a second system that is IDENTICAL except for video card (Geforce 4 Ti vs GeForce 2 MX) and bios revision. One works perfectly, the other frezes on startx.
The one that works has bios 1.001E, the one that freezes has 1.03. I haven't tried the brand new 1.04 bios to see if it fixes it.