The use of a projector is one of the methods used to display a visual feed back on the table. For the long run any video projection device should work but there are many to pick from. because this is the most expensive piece of equipment you should do a little research on it.

Type of display

There are two main display types DLP and LCD.

LCD
Liquid Cristal Displays are made up of a grid of dots that go on and of as needed These use the same technology that is in your flat panel monitor or laptop screen. Theses displays are very sharp and have very strong color. The negatives are that they have a screen door effect and the color tends to fade after a few years.

DLP
Digital Light Processing is a new technology. These work by the use of thousands of tiny mirrors moving back and forth. The color is then created by a spinning color wheel. This type of projector has a very good contrast ratio and is very small in physical size. The negatives are that DLP can have a rainbow effect that is annoying to people and normally has a slower response rate.

Brightness

Brightness is measured in lumens. The higher the number the brighter the projector and the brighter the setting can be. In a home theater setting you alway want a brighter screen but in a table setup it differs.

With the projector close to the screen and the screen size being smaller the projection can be too bright and give you a hotspot in the screen. This hotspot effect can be dazzling after looking at the screen for several minutes.

Throw Distance

One of the biggest limiting points of a projector is the throw distance. This is the distance that is needed between the lens of the projector and the screen to get the right image size. For example in order to have a screen size of 34in, then you may need to have a distance of 2 feet between you projector and the screen.
Check to see if your projection image can be adjusted to fit the size of the screen you are using. Here is a link to a very good calculator for most projectors.

In case you want to make your multi touch display into a boxed solution, you might want to use a mirror to redirect the projected image. This provides the necessary throw length between the projector and screen while allowing a more compact configuration.

Aspect Ratio

The aspect ratio is a measure of image width over image height for the projected image. for example if you have a standard television then your aspect ratio is 4:3. If you have a wide screen tv then you have a 16:9 ratio.
In most cases you want to go with a “normal” 4:3 ratio projector but the ratio depends on the use of the interface.