Multiple GPUs problems How to use different GPUs in one board

In my workstation, I have three GPUs installed. Two GeForce GTX 275 and one GTS 250. While I try to install the drivers for them, I encounter some problems. Either driver for GTX 275 or GTS 250 would work. The system cannot load X-desktop. I can merely work in the commandline mode which is quite inconvenient. So what’s the resolution?