This isn't a limitation of Linux, but a limitation of the current drivers. With the most recent releases of QuakeWorld and Quake II, the 3Dfx miniport mentioned above is available for Linux. Also, in Quake II version 3.20, the OpenGL function handling code was completely rewritten, resulting in significant speedups. GL Quake speedups. It allows you to create many virtual screens in one tty and switch between them.
Screen comes with most distributions.
You won't see much as you do these things, but be assured, something is happening. Screen and your servers are still running, but they're no longer visible in your terminal window. You can logoff now and your processes will continue to run normally. That's all there is to it. Do the reverse to release it.