Can anyone recommend a quality projector, or explain some tech specs?

Our department at work is looking into purchasing a new projector for our meeting office. The ones we can buy in the campus catalog are Epson projectors, which I’m assuming are of good quality since they’re listed there. Thing is I’m not sure about some of the specs listed beside each one. What’s the difference between WXGA and XGA? What is “all HDMI connectivity?” It should be simple for all to use, so I like the fact that some (but not all) of the ones I’ve seen have (bonus) USB plug-to-play for both Macs and PCs. Price range is about $500 – $800 and the room it needs to project across with a clear image is about 20–30 feet.