OEM computers are pretty limited in what can be upgraded and laptops are even more closed off than desktops. According to Acer's website these are the only CPU's that will work.
Intel Celeron mobile processor 530
Intel Celeron mobile processor 540
Intel Celeron mobile processor 550 (1 MB L2 cache, 1.73/1.86/2 GHz, 533 MHz FSB).
IMO none of them are worth it. Your laptop seems kind of old I would just get a new one.