Difference between 720p and 1080p projectors?

I've had an Optoma H79 projector for 5 years and it still works great, but I'm wondering how much video quality difference is there between it's 720p resolution and newer projectors offering 1080p. And is there a very noticable visual difference/improvement or essentially the same?
Is a 1080p picture roughly 20% better than a 720p?
Is this a great example or what.Is there any programing in 1080P.1080i is all thats used.the hd programing, the cameras are 1080i.At least that was my understanding.Good luck
look at projectorcentral.com for answers to many technical questions regarding projectors
Most HD cable, satellite, and over the air signals are broadcast in 720P (1080i) not 1080P. The most common source for 1080P is the Blu-ray DVD format. So, on broadcast HD, you won't see much difference between a 720P or 1080P projector.
I may be wrong,but I think a 1080p has around 2,076,300 pixels to make a picture,720p has around 921,600 pixels.That is 2X the pixel count.I don't know how much of a percentage increase in a sharper picture that would be.200%? But the larger the screen,the more you could tell how much better 1080p BluRays are.The way I understand,we won't be seeing any 1080p broadcast for years due to limited bandwidth.Who knows if there will be any other 1080p sources besides BluRay,computers,and video games.Don't hold me to this info.
720p looks about like a good 35 mm print in a movie theater. 1080p sourced from Blu-ray to a 1080p display looks like a clean window onto the real thing.

720p IS NOT the same thing as 1080i. 1080i actually has 1080 discrete lines of resolution; it's just that it takes two 60Hz cycles to see them all. Still, you're getting a display of 2,076,300 pixels. A 1080p display upconverts 1080i to 1080p and you really get 1080 lines of resolution. Although a native 720p LCD/DLP/Plasma display can accept 1080i input, it downconverts it to 720p, which gives you 921,600 pixels.

In the case of a front projector, I would think that a 7 foot screen would show a dramatic increase in sharpness with a 1080p projector over 720p, especially when sourced from either 1080i broadcast or especially 1080p Blu-ray. Once you experience 1080p from source to display, you'll never want to return to anything less.
Thanks for all the info guys; just when I think I'm understanding more about this hobby, I'm humbled again. If later this year, I was to upgrade to a 1080p projector for Blu-ray DVDs, what projectors would you recommend for under $2,000?
I upgraded from an Optoma H77 (which I loved) to a Epson
7500ub last year. Also as part of this upgrade, I went
from a Pioneer Elite SD dvd player to a Pioneer elite blu-ray player. The difference is honestly night and day. The Epson ($3000) is so superior to the H77 ($5750) four years ago that it really is worth getting into 1080p as soon as you can afford it. With 1080p you will never go to a movie theater again.
you can buy a 720p projector to save some bucks, and then consider upgrading when 1080p projectors become affordable.
The Optoma HD20 is $999, and is an amazingly good 1080p DLP projector for the price. Also very bright--I can use it in a room with quite a bit of light. The 1080p projectors tend to best the 720p in a variety of ways, not just resolution. But even for lower resolution sources you will get a smoother picture. Blu-ray looks absolutely as good or better than any picture I have ever seen in a theater.
A 1080p pj will certainly look better than a 720p pj when it comes to br. Hdtv is another story. It may or may not look better depending on broadcast and your lighting conditions remembering that with ambient light you need more lumen output. I have a new Epson 8100 and could not be happier. It's not my dream jvc but it's kickbutt for 1.5k