homemade EFIS system and EMI

RFI is definitely a problem, and fixing RFI problems can be a black
art.
You don't actually have to have _any_ chip running at the problem
frequency. Square waves have a lot of odd harmonics. So, something
switching regularly at 1/3 or 1/5 the problem frequency can cause
problems. That something can be a software routine in a chip running
much faster than the problem frequency.

You might want to reconsider your preferred form factor. PC
motherboards are amazingly inexpensive, but they're relatively large.
Have you considered the PDA form-factor? Smaller, less power-hungry,
built-in display, and most of the RF problems will be already handled.
Should still be able to handle the audio and display functions.

If you prefer Linux over PocketPC or Palm OS, I know the Zaurus PDA has
had Linux ported to it.

Glider pilots use PDA's as glide computers, so they're known to run
without problems in systems with aviation band radios. If you're
sending the data from the 8051 via the serial port, this is how the
glide computer PDA's interface with the GPS and/or vario systems.

Downside is, RS232 ports seem to be going away in favor of USB, and USB
OTG (where the PDA can act as a host instead of a peripheral) seems to
be very slow in coming in. OTOH, older PDAs with serial ports are
dirt cheap on EBay.

And, for 8051 chips, the Dallas 89C440 is a pretty cool little chip.
It can run internally at up to 4x the xtal frequency, 1 machine cycle/
clock instead of 12.
Flash, so you can reprogram it in-circuit. Built-in loader so all you
need to program it is a PC and a serial port. 32K bytes of program
space. (64K on the 89C450)
2K bytes of xdata memory on chip, and the usual 256 bytes of the 8052.
There are C compilers for the 8051 architecture, so you're really not
limited to assembly for development. About five bucks a pop, small
quantities.

Tim Ward