The lowest resolution you're likely to find in a projector is SVGA, which is 800 x 600 pixels. Use this type of projector if you're showing mostly PowerPoint slides and simple graphics. XGA or 1,024 x 768 resolution is fairly standard in projectors and is better for displaying spreadsheets or detailed graphics. Less common is SXGA, which displays at 1,280 x 1,024 resolution. The brightness of a projected image is measured in lumens, or light output.
More lumens means more brightness.
More brightness means a crisper image even in the face of light. If you'll be presenting in a small room with dark or dim lighting, a projector with less than 500 lumens should be adequate. For a normal-sized conference room with dim light, look for at least 500 to 1,000 lumens. Large conference rooms with normal lighting require at least 1,000 to 1,500 lumens of brightness. And if you give monster presentations in auditoriums, you'll need 1,500 lumens or more, regardless of lighting conditions.