Introduced in 1993, the Macintosh Quadra 840av is the fastest 68K Mac before Apple moved to the PowerPC platform. The CPU clocks in at 40MHz and is a full 68040 compared to the 040 LC and Centris series which was typically without the FPU. These powerhouses can be upgraded to a maximum of 128mb RAM and support three Nubus expansion slots. S-video input and output are also standard additions to the logic board. The special DSP chip can be used by supported audio/video applications to increase performance.
The Quadra 840av supports System 7.1 through MacOS 8.1. The system does have one major drawback, which is the inability to run the Apple UNIX (A/UX) software due to the OS's incompatibility with the DSP chipset. A Quadra 800/950 must be used for A/UX.
My personal Quadra 840av setup includes a 2gb HD, Zip drive, 17" Multiple Scan display, and a full 128mb RAM. It runs MacOS 8.1 quite nicely.