The ones made with overhead projectors usually turn out to have horrible picture quality and are generally not bright enough, plus the lamps only last for a couple hundred hours.
Check out lumenlab.com to find out how to make a real projector. The ones that some people turn out from here end up having picture quality rivaling $5,000+ projectors for around $400, plus the bulb lasts for 20,000 hours instead of 3,000 and costs about $30 instead of $300.
I'll be using a 15" CMV CT-529A 15" panel, 500:1, 16ms response, no LLC issues (the flat cables inside the monitor can get in the way, this one doesnt have the problem), and is cheap.
Buy the guide and the lens kit from lumenlab, find a good ballast, bulb, and LCD and make a wooden (or cardboard) box to put it all in.
Tomshardware is never a good place to start looking on where to start a project.
http://www.lumenlab.com/forums/index.php