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Cyrix designed MediaGX as a CPU used for budget systems. The traditional northbridge and graphics/sound capability are combined into the CPU. The graphics capability is equivalent to a slow-slow PCI video card, and can be set to either 1.5 or 2.5 megs of video RAM, shared with the system memory. The sound capability is equivalent to a Sound Blaster 16; it has FM synthesis, and full-duplex 16-bit DSP. MediaGX is paired with Cx5530 southbridge, which has no IDE DMA driver under Windows and no IRQ steering capability. Overall, MediaGX is a weak solution, but if you got it for free or cheap, hey, it can be useful.
MediaGX has the same PPGA package as a Socket-7 CPU and runs at 3.3V and 66MHz bus speed. You can *NOT* however use a MediaGX CPU in a regular Socket 7 mainboard. The mainboard must SPECIFICALLY mention that it is designed for MediaGX. Likewise, you can't use any regular Socket-7 CPU in a MediaGX mainboard. Socket-7 and MediaGX are **MUTUALLY EXCLUSIVE**.
There are two types of MediaGX CPUs. The MediaGXI is similar to a classic "Pentium" processor, with 16KB of L1 cache. The MediaGX-m is similar, but adds MMX instruction support and clock speeds up to 266MHz.
Performance of this chip sucks. Don't expect to use true color or over 800x600 graphics without major performance deficit. Also, don't expect to be playing any games on it, the integrated video coupled with famous Cyrix slow FPU won't get you much of anywhere. The lack of IDE DMA drivers is another thorn in the side of performance; the two IDE controllers perform as your run-of-the-mill ISA IDE controllers under Windows. Terrible!
Linux is a better choice than Windows for a system using this CPU. The less overhead of Linux kernel and drivers, and less dependence on raw throughput, will show a vast difference in overall performance. In addition, Linux has DMA support for the Cx5530 IDE controller, so disk throughput and CPU usage will be improved greatly. A MediaGX system may make a very decent (and cheap!) Linux box, especially if you have no need for the graphical display or sound functions.
Cyrix (or anybody) no longer provides Windows drivers for the MediaGX video and sound components. Here are the last revisions I was able to archive.