Can I mount a projector upside down? I read in the manual that it may affect bulb life if the projector sits at a tilt but what if it is upside down but level?

Currently I have a 54" rear projection TV. I have been very happy with this TV but, the time has come for better quality. I have been looking at LCD and Plasma TV's but, not very happy with the cost of a large screen. So I am faced with a 44" LCD for around $1300 - $1700 or plasma that would require a loan against my home. To get the most bang for the buck it looks like I am looking at projection. I am looking at the NEC VT575 due to its short throw ability. I am looking for a LCD projector around the $1000 to $1300 mark and this was best I could find looking at specs only. My wall to wall measurement is 12ft with a viewing distance at around 10.5'. On the projecting wall I have available 84"W X 96"H. To my understanding width can only be 1.5X or .66 of viewing distance. Which I get 84' wide. Which I don't want I want a 68" wide 16:9 screen. Projection calc states a projection distance of 10.1' using eco mode. I was looking to build my own screen with blackout cloth for the time being. Is this a good thing to do ? Questions. I do have windows in the room that we have blinds over and often keep open. I do like to watch tv when the blinds are open. Is this going to be possible ? This projector is a 1024X768. What is HDTV ? is it 720p and/or 1080i ? This is where I get confused with resolution and 720p and 1080i. I have Directv and no HDTV. I watch alot of DVD's but, want to have the ability to get the most detail as possible. Thank you very much for your help