The Treo 650 can use a 4GB SD card. I am using it now and it works fine. You have to flash the ROM, but it's not difficult. I can point you at instructions if you like, but basically you replace the FAT16 driver with a FAT32 one someone got from a LifeDrive (I checked using a friend's LifeDrive). The new driver is faster too!

Another nice thing about the 650 is that the battery is replaceable. You could get a new one if your present one has lost its oomph.