Now that you have your pictures picked out, it is time to consider the next step. Framing all of the images.
Anyone who has had anything framed knows one important thing; it is more expensive than you might think. Professional framing, and choosing a frame and mat to best compliment an image is an art unto itself, to say nothing of the precision used in assembly. Then there is non-reflective glass, UV glass, UV plexi, museum glass and so on and so on. This is all fine and well, but if you are looking at having 20 to 30 images framed (and I have done walls in excess of 100), the cost can be hard to justify. This is where pre-made frames are handy and stores like Pottery Barn, Ikea, Crate and Barrel and Dick Blick come in. All sell basic frames, typically with plexi, in standard picture sizes at a fraction of the cost of a custom frame.
For a photo wall installation, your frame choices will come down to either making all of the frames the same or mixing styles and colors. The basic guidelines for mixing black-and-white and color photos also applies to frames. All the frames need not be identical (although it is a nice look), but if you choose to vary the frames, try to have a distribution of them (i.e. do not mix 2 white frames in a sea of brown frames). I will admit it is difficult mixing very light and very dark frames together (i.e. white and black frames), but it is possible. You can make the overall image stronger by the variety of frames you bring together. A wide mixture of frame styles can also add a great deal to the arrangement. These are extremes; a very nice approach it to gather a variety of black frames that differ in width and style (square, carved, rounded, etc.).