Ah the good old RT2500... I have one of those in my laptop. Took me ages to get the damn thing working.
However.. it does, with 64-bit WEP, so I imagine 128 will be the same.
I use a script with the wireless tools (on Slackware, but I assume it'll work ok on debian).
My laptop calls it ra0 -- change that to whatever debian calls it (if any different).
ifconfig ra0 up
iwconfig ra0 essid any
iwconfig ra0 key restricted
iwconfig ra0 key <your WEP key here>
iwconfig ra0 channel 11
iwconfig ra0 rate auto
The key line does need to be in there twice, so it's not a typo
Hope it helps.