If I copy the schematics for http://www.sparkfun.com/products/9734 , but swap out the chip antenna for a SMA duck antenna or wire antenna, do I need to change anything else? The reason I ask is that some other RF designs have a capacitor in series with the antenna but this schematic does not.
The balun used on that board is very likely designed to drive 50 ohm antennas, so you should be able to remove the chip antenna and replace with an SMA connector and antenna.
If you use a wire antenna, you’ll have to add some components to give a 50 ohm impedance (required to maximize performance), so I recommend you stick with ready-made antennas.
I really want to be able to use a wire antenna, I know I’d have to cut it at about 1/4 of the wavelength and then trim it down until it’s the best length. What components do I need to add?
If you have radials at 45 degrees to the horizontal, a 1/4 wave antenna will have an impedance of about 50R. Over a ground plane, it will be lower IIRC, but probably won’t cause problems.