First decision is LCD or Projection. 6' x 4' screen gives you about an 85" screen, and while it is significantly larger than the 52" lcd, I don't think it's enough of a jump to be worth the cost. I've got a 120" 720p projector and a 37" 1080p lcd. IMO pProjectors just aren't good for everyday use, bulbs are expensive and duty cycle is limited (they're not meant to be run continuously, the longer it's on each time you use it, the shorter it's bulb life). LCD's can be used the same way you use any TV, you don't worry about if you accidentally left it on. Also, you're never going to match the image quality of an LCD with a projector (at least one that doesn't cost 10k or more). If you're viewing area limits your screen to less than 90 or 100 inches... I'd recommend you get the LCD.
Second... if you do go the projector route, eraser board would make a TERRIBLE screen... you want a flat finish not a highly reflective glossy finish. What you'll end up seeing will be the superbright center point in the middle of the screen with light falling off dramatically at the edges.
ALSO, if you decide to go with a projector I strongly recommend getting a decent screen at the same time. There are a lot of inexpensive screens out there... but you will get what you pay for. A good quality screen will cost in the vicinity of a grand. Just suck it up and pay it... why spend $3k+ on a 1080p projector then cripple it by using a crappy screen?