Like many vendors, Belkin implements bridging based on the Wireless Distribution System (WDS) defined in the 802.11 standard. WDS uses standard 802.11 wireless to forward traffic between APs, as an alternative to Ethernet or another wired distribution network. In effect, WDS turns a pair of APs into a pair of wireless bridges or repeaters. An excellent tutorial on WDS and how to configure it can be found at Tom's Networking.
WDS is based on the 802.11 standard, but is not fully specified by that standard, so WDS links often work only between same-vendor APs (or APs using the same chipset). However, WDS uses not only the same frequency band, but exactly the same radio channels, as other 802.11 products. In this case, Belkin 54G bridges transmit on the 2.4 GHz ISM band and channels defined for use by 802.11b/g. There are a few nice illustrations of WDS and channel assignment dependencies for bridges and repeaters in this Proxim document (PDF).
According to Belkin specs, the 54G router has an indoor range of 200-300 feet and an outdoor range of up to 1800 feet. Belkin does not publish specs for bridge-to-bridge mode, but with AP-attached dipole antennas, signal is distributed in a roughly circular pattern around the AP. That means you are looking at perhaps 150 feet from the AP in a given direction (indoors). From what I can see, this router's dipole antennas are not intended to be removable / upgradeable, but here is a forum post that I found on how to replace factory-installed antennas for another Belkin wireless router.