The pre-built XF86Config file that Patrick put together, as it states in the header is built to work with just about any graphics card and some base resolutions. Unlike every other distro on the planet, it is not the product of a probe of your actual hardware. Its pretty cool actually that it worked on both machines I installed it on, and whoppingly new and old monitors!
I have the same card, hooked up to a 19-inch VESA capable monitor, and ran ./usr/X11/bin/XFree -configure
Then I edited the outputted file adding in, under the resolutions:
ViewPort 0 0
Also, as the ./Free probe doesn't get all the fonts right, under the section Files:
As well as adding the module "freetype"