measuring voltages

Hi all,
Wanting to simultaneously measure 26 different voltages, at various points on a circuit board (when the input voltages are supplied), using a test fixture, the results to be to be displayed on a computer monitor. Can anyone suggest a suitable microcontroller chip, and possibly how best this task could be accomplished? I was thinking the atmega2560 in tandem with an atmega328P perhaps. Anything better?

How important is the "simultaneously" aspect? The ATmega2560 and ATmega328 only have 1 A/D converter.

Thanks for the response James. Just testing a PCBA inputting 5V dc, +/-12Vdc, and one 5V p-p 4MHz to supply lines through a 25 pr. ribbon to an input connector, and looking to measure the outputs obtained through at a 26 pr. connector port for the board to check for correspondence with a simulation run for the circuit. I know you can measure up to 5 analog input voltages with the atmega328P uno, and was just looking at the specs for the 256 which has up to 16 analog voltage input measurements. So I guess two of these chips might suffice. How best do you think I could measure these? Actually, it seems I may have to check on 29 different voltages at various points on the board.

Why not use an analog multiplexer? Again this boils down to the question what "simultaneousyl" means. Notice that no matter what you do on a 8bit CPU you will be facing a hard time to read more that 8bits at a time anyway.

Thanks for the suggestion. Must say that I'm the proverbial newbie when it comes to using multiplexer chips. Is there any one in particular that you'd suggest? By 'simultaneously' I mean this: three connectors from the test fixture are attached the board; one is for input voltages, the other mainly supplies the voltages for the power buses of the board, and the third is for the output signals to be measured given simple inputs at any one time.