If I wanted...

Just a bare-bones arduino Mega (atmega1280) what would I need?

the atmega, the ftdi usb to serial converter (or any usb to serial converter ic), the external crystal and both caps, a step down converter or voltage regulator depending on input source

That's what I've got so far.. Is there anything else I'd need?

Might want to add a pull-up resistor (10k) for the AVR reset pin. Also a series .1 mfd cap between the FTDI DTR pin and the AVR reset pin will let the Arduino's IDE auto-reset function work for uploading new programs, otherwise you need a manual reset switch. Also a few caps for the regulators input and output terminals and .1mfd bypass caps at the voltage power input pins (Vcc, Avcc) at the AVR package.

Lefty

Thanks. I've not used caps in any projects before, so could you explain why they're so necessary? I know the general purpose of a cap but I'm not entirely sure why they're needed. That's one of those electronics bits I've never familiarized myself with or bothered using haha.

Thanks for the tips. I'm assuming mfd is microfarad which is uF?

In this case the caps are used to route AC noise to ground, it's a noise reduction thing. In the case of voltage regulators, they can oscillate without proper bypassing. Yes, mfd = uF.

Here is a schematic you can read to see where caps are utilized:

http://arduino.cc/en/uploads/Main/arduino-mega-schematic.pdf

Lefty

Hmm. Okay thanks so much.