um, i don't really think the ram module is the problem. I have linux and mac os x installed on this laptop. The linux is 32 bit, but i enabled the pae in the kernel, so i can use the total 4 gig without any problem. I can also use Mac os x with the total 4 gig ram.
I think i will just repair the Os. Anyway, thanks for your quick reply guys, really appreciate it.