D-Link sells a number of external antennas that can be used with your DWL-2000, ranging from shorter-range indoor omni antennas to longer-range outdoor directional antennas. Although actual distance varies with RF environment, D-Link publishes this chart to enable comparison between its products:
For example, the ANT24-0401 and ANT24-0500 are both indoor Omni-Directional antennas with a 360 degree horizontal field. The ANT24-0401 (3 dBi) has a 63 degree vertical field, while the ANT24-0500 (5 dBi) has a 32 degree vertical field. The ANT24-0500 compresses available output power into a flatter "donut" to reach farther: the ANT24-0500 has a range of 1650 feet, while the ANT24-0401 has a range of 1312 feet.
Compare this to the ANT24-1400, a 14 dBi Directional Panel Antenna with a 30 degree horizontal / 30 degree vertical field, and the ANT24-1801, a 18dBi Directional Yagi Antenna with a 15 degree horizontal / 15 degree vertical field. These outdoor High Gain Directional antennas not only flatten the coverage area, but narrow the horizontal beam width to reach much farther than Omni antennas. According to D-Link, the ANT24-1400 reaches up to 1.875 miles, while the ANT24-1801 reaches up to 3.125 miles.
When selecting an external antenna, you'll need to consider environment, distance, and required coverage area. The higher the gain (dBi), the more focused the coverage area. To get a feel for the coverage areas produced by these antennas, download the D-Link antenna Install Guide from http://www.dlink.com/products/support.asp?pid=56&sec=0#quickInstallGuides.