we have a Powerbook G4 and intend on upgrading the RAM. We opened the slot on the back of the device and found a 256MB PC2700 CL2.5 DDR module. So we need to replace it with another device that has more memory and the highest it finds is 512MB. Also, that one has 333MHz (which we didn't find out what it has on the current module). Are there 1024MB (1GB) available? Thank you very much!