By "flash card" I'm assuming you mean a removable memory card. I don't know about the palm tx specifically, but I have a Clie SJ33 that uses memory stick media. The palmOS on mine has a utility called memory stick export. With that the Clie works as a card reader and I can mount the memory stick as sda1 (or whatever). Browse yours to see if you have something equivalent.
Very important: First, make sure you have handspring visor support in your kernel, either as builtin or module, otherwise your palm device won't be detected at all.