Gigabyte GA-7VT600 Ram

Hi all,
Brought a 2 gig ram chip. After removing old 512 meg chip and replacing it with 2 gig, the computer fails to boot up, the errors are random, typical of ram errors, on replacing the 512 meg chip, all is ok again. I plugged the 2 meg chip into my sons mobo, which is a Soltek SL-75FRN. The chip works OK but windows only sees 512 meg. Can anybody advise please? Ideally i would put the 2 meg chip on the GA-7vt600, but if not possible i would like it on the Soltek.
Thanks in advance for any help.