screen resolution problem with xorg
So after a successful install of Arch I installed the vesa video driver and then Xorg. I worked through the Xorg page on the Arch wiki and it seems to be configured properly, but when I "startx" into xterm or lxde or whatever, the screen resolution has failed to set itself to what I put in my 10-monitor.conf. Instead the resolution is tiny and the picture resides within a black border around my screen. I have never encountered this problem in any other Linux distribution.
My card is an ATI Radeon HD 4870; my display's max resolution is 1920x1080. In addition to the vesa driver, I have tried the open source ati and radeonhd drivers with no success. What is different about Arch Linux that causes my hardware to not be detected properly?