1. Two DIMMs should make the most out of DDR memory, right?
Yes, unless your laptop can hold 4 SO-DIMMs.
Dual Channel must be faster than my current 1 DIMM, right?
faster throughput, yes, but it is unlikely that your laptop supports dual channel mode.
2.what's the fastest memory my laptop can accept without any compatability problem?
You will need to get that info from Toshiba.[/quote]
Is it wasting money to get any memory faster than PC2700 or I should get PC3200 to make my whole system faster?
It depends. The laptop will run ALL the memory at the speed of the slowest memory SO-DIMM. Going from your current PC2100 speed to PC3200 speed is a big jump. Going from PC2100 to PC2700 is a smaller jump. Here's what I would do:
1) If your laptop can run the memory at PC3200 (DDR-400) speed, I would buy 1 new 1GB PC3200 SO-DIMM, and sell the old 512MB PC2100 SO-DIMM.
2) If your laptop's maximum memory speed is PC2100 or PC2700, I would just buy 1 new 1GB SO-DIMM (or 1 new 512MB SO-DIMM) and add that to the computer. Remember that faster memory will run OK at slower speeds, so buy PC3200 speed SO-DIMMs unless the slower SO-DIMMs are much cheaper.