You should know the difference between LCD and DLP, for one. I prefer DLP over LCD because I think the image just looks better, and has less "screen door effect". (Screen door effect is when you can see the lines separating the pixels, if you get close enough, like looking through a screen door to see the image--do a search and you will probably be able to find it...)
I also think DLP in general has deeper blacks and better color, but I'm sure there are people who prefer LCD who might tell you otherwise.
(DLP uses an array of tiny mirrors to reflect projected light after travelling through a color wheel. LCD projects light through an LCD panel. By the way, the color wheel makes some people tend to see "rainbows" (the rainbow effect) in DLP projection--on a high contrast scene (white text on a black background), darting your eyes causes a brief flash of color, like a brief rainbow--I can see them, but I am not bothered by them, and I don't even really notice them anymore.)
Although you can certainly get by with an SVGA projector (800x600), you could probably get an XGA, or even WXGA (the "W" stands for "widescreen"--which means the pixel panel has a widescreen aspect ratio) fairly inexpensively. These just give you increased resolution of the pixel panel, providing a larger amount of smaller pixels on the screen.
I would look into demoing some projectors to get a feel for what they can do. Although there are a host of factors to consider in your projector, if you break it down, you really only need to look at three things:
3. Contrast ratio
These factors are the keys to getting a decent projector.
1. Brightness. I already talked about brightness--I would recommend a bright projector for outdoors (not only if you have stray light, but because you are spreading your light source onto such a huge canvas--my screen is 16 feet by 9 feet. The bigger the image, the less light per foot.
2. Resolution. If you are only using the image for outdoor viewing, resolution is not that big of a deal. In fact, while 720p and even 1080p projectors are all the rage (and you pay a lot more for them), my little SVGA projector (coupled with an anamorphic lens, which I won't go into now, but can provide more info if you don't know what this does) throws out a really good image, and while it isn't HD quality, it is pretty stunning I believe.
3. Contrast ratio--I haven't talked about this, but the higher the contrast ratio, the deeper the blacks and better the overall picture.
The quality of the final image is directly related to these three factors. Hope this helps.