The Linksys WRT54GL is a very common wifi router, because it was the first router that could be flashed with community based firmwares like openwrt.
It is quite old, slow, and only has 16Mbyte of ram, and 4Mbyte of flash. If you intend to buy new hardware, this model is not recommended. There are now more powerful and cheaper routers.
It however does the job very well, if you don't expect to have more than about 15 simultaneously connected users.
Download the bin file if you flash from linksys firmware, or use tftp method.
Download the trx file if you flash from older openwrt.
Simply go in the router's firmware update page, and upload the previous bin file.
This router is safe to flash, because you can reflash it even if you made a mistake with the boot-wait tftp method.