Well for your purposes, i think the 328 should suffice.
What you made is all quite simple if you just go through the playground a bit and find what your looking for.
As for the pir, theres some sample code here; http://www.arduino.cc/playground/Code/PIRsense
yeah... i have seen everythng in tha playground are. but there is everything in parts.
like i know how to use the pir and buzzer seperately. but i dont know that how will it work collectively. what i mean to say is that i know how to make the buzzer work when the pir senses motion. but what i dont know is that how to display an alert on the lcd at the same time.
but what i dont know is that how to display an alert on the lcd at the same time.
Well, that's the bit you should concentrate on.
If you know how to sound a buzzer in response to a stimulus from a PIR, then the same principle applies to displaying messages.
AtMega vs 328,
well 328 seems to be fine if your alarm system only wants a few zones.
1 out for the buzzer, 1 out for the serial display, and still plenty for input zone detection.
i was just going through the manual on the website. i am a bit confused.
can i delete some program from the memory of the chip? and once i have uploaded the program do i have to lock the atmega chip using burn bootloader?