I've had good success with a 4g A-Data SD memory card on my Palm TX. I have not been terribly satisfied with the speed, particularly when using Backupman for my daily backups. I have had to schedule them for 4:00am due to the amount of time taken for backups (anywhere from 15 min. to 30 min). However, the card performs the task of keeping lots of videos and music onboard, so I can't kick.
Just for your info, the card was advertised as "150x" speed. I can get much more speed out of a typical 1g SD card, however the music plays just fine using PTunes 4.0, and the movies work without lags or interruptions using TCPMP.
My TX still needs at least one soft reset each week due to some odd lockup, but I'm running loads of apps, so I consider this to be acceptable.. and it never seems to be the fault of the 4g SD card. I've even considered buying a second one to have on hand. Also, be sure to get yourself a pocket-sized SD card reader to use when loading up the 4g card from your PC. Don't torture yourself by trying to use the Palm Desktop to add music or videos to the card.. it's just ridiculously awful speed-wise. I recently tried to use "Sync" from within the new Windows Media Player by way of Softick's Card Export on the TX for my music.. took hours to complete!
Just my $.02