I'd use foam swabs and rubbing alcohol, erasers can leave rubber residue on the contacts and cotton swabs like Q-tips can leave cotton fibers. Also when reseating the sticks ensure the clips lock themselves in so fully seated. Sticks shouls be arranged 2GB-1GB-2GB-1GB going from CPU. Look in MSCONFIG and see if memory is being limited
go to START > type MSCONFIG > Enter > BOOT > ADVANCED > ensure Max Memory is checked