Firstly waht PSU do you have, as in make an model (all 400W are not equal). Have you had the chance to try the card in another machine? Have you had a look at BIOS update here:
http://support.asus.com/download/dow...&model=A7N8X-X
Nothing GPU specific is listed but you never know, it may also solve the problem of you having to manually set the CPU FSB as there are updates for that! Do you have fast writes on and AGP set to 8x together with an aperture of 128MB? If all else fails you might like to try a complete OS reinstall with the latest nVidia drivers and see how that goes.