Xpire

I recently bought some ram for an old computer lying around the house. I'm not really a 'pro' at computers so .. yeah. Anyhow, i checked my motherboards manual bla bla. found that i should buy a DDR PC2100 266. I already had a stick of 267mb running inside and bought a 512mb pc2100. after putting in the 512 stick. the computer read 700+mb of ram, but after around.. an hour or so.. and usually when running games or running big things, the computer would freeze or restart for no reason. i decided to take out the old 256 mb ram, and leave the new one in, this worked fine , but was the MINIMAL amount i needed =\ does anyone know why this happens? the two sticks of ram are different brands, ive noticed that there're.. 'chips' or something on both sides of the new one, while the old stick only has it on one side.