I am giving a little answer, but even then I will ask which exact model in the A900 series he does have
Current A900 series laptops
Intel T2390 details
The specified Bus speed given in here is 533MHz,
you cannot put a faster RAM.
faster than proccy Bus speed or max supported bus speed of the mobo.
chipset you got here is GM965, which I hope supports upto 800FSB
GM965 chipset details
Download
CPU-Z on his laptop and run it. It will give you exact details of the proccy and mobo and bus speed of RAMs all info you need.
Final verdict, get 533 FSB DDR2 RAM and not 800 FSB DDR2 for the laptop, besides that the A900 series supports max 3GB RAM