wireless from mac to projector

You are not providing much information, but basically you would want to create a wireless network with the AirPort Express Base Station (AX), and then, join this network from both the Mac and the inFocus projector.

Here's how to set up a wireless network with the AirPort Express Base Station (AX):

AirPort Express Base Station Setup (AX) w/High-Speed Cable Modem

Perform a "factory default" reset of the AX
o (ref: http://docs.info.apple.com/article.html?artnum=108044)

Modem/Router Power ReCycling
o Power-off the Cable modem, AX, & computer(s); Wait at least 30 minutes.
o Power-on the Cable modem; Wait at least 15 minutes.
o Power-on the AX; Wait at least 5 minutes.
o Power-on the computer(s)

Setup the AX
Connect to the AX's wireless network, and then, using the AirPort Admin Utility, try these settings:

AirPort tab
o Base Station Name: <whatever you wish or use the default>
o AirPort Network Name: <whatever you wish or use the default>
o Create a closed network (unchecked)
o Wireless Security: Not enabled
o Channel: Automatic
o Mode: 802.11b/g Compatible

Internet tab
o Connect Using: Ethernet
o Configure: Using DHCP
o WAN Ethernet Port: Automatic

Network tab
o Distribute IP addresses (checked)
o Share a single IP address (using DHCP & NAT) (enabled)

Once you verified that you can get Internet access for all of your computers, you should secure your wireless network.

So you want to use the projector as the monitor for a computer, but you want to use wireless to join them?
You won't be able to do that using an Airport network. Computer networks can only pass data that's been specially formatted. They can't pass video streams in a form that a monitor or projector can understand.

To do this you need a video sender - a device that accepts a video feed from a TV, DVD or satellite box, then sends it wirelessly to a receiver which you connect to your TV or projector. In your case, you would need an S-Video output on the computer to connect the sender, then connect the receiver to the S-Video input on your projector.
Problem is, video senders don't work as well as cables. You'll get some degradation of picture quality (noise) and possibly some interference from other wireless devices (dropout).

If you want to watch video that's stored on a networked computer, with decent picture quality, you'll need to connect a computer or a network media player directly to the projector using a HDMI or DVI cable.

What do you currently have connected to the projector?

I don't use a projector, just a 46" Panasonic plasma TV. I too want the best picture quality.

Why not get an HDMI switch? Put that on your equipment rack with the receiver, DVD and Mac. Connect all the devices to the switch with HDMI cables, then run a single HDMI cable to the projector.

I use a remote controlled 4 input HDMI switch that I got from eBay for about 30GBP (50 USD/Euro). Works perfectly with my HD media player, DVD and Mac Mini. For the Mini, I used a DVI-HDMI cable, also very cheap from eBay.

If you want to stream video from other computers, you can do that via the Mac using wireless n or ethernet. Wireless g is too slow for HD video.

You could use any computer you like connected to the projector, as long as it is fast enough to to play the video. A Mac Mini is ideal as it's small and quiet. The current models are all fast enough to display HD video. Even my old 1.5GHz Core Solo (the slowest Intel Mini) copes perfectly with DVD rips.

Once you've got a computer connected to the projector, you can store your video files on any computer or storage device on your network. A Time Capsule or Airport Extreme, with a USB HDD plugged into it, is ideal for this.

As for controlling the Mini, you could do this using a laptop and VNC software to remotely access it, or simply use a wireless keyboard and mouse, or a wireless keyboard with a built-in trackpad. You can even control many functions using the Apple Remote (supplied with the Mini), or a Universal IR remote like a Harmony.

There are some "wirless" projectors around now. For example:
http://www.epson.com.au/downloads/pdf/EpsonEMP1700Series.pdf

However, the wireless projectors that I have looked at do not have the latest 802.11n Wi-Fi capability so I doubt if they are capable of high quality video transfer. But I would be happy to hear from users who have got this working well.