ATTEMPTING TO UPGRADE THE 256MB

Question

ATTEMPTING TO UPGRADE THE 256MB RAM ON MY EMACHINE 540 I PURCHASED A NEW PC133 256MB RAM.AFTER INSTALLING IT THE MACHINE STILL WORKED OK BUT STILL ONLY HAD 256MB RAM NOT 512MB. BOTH OLD & NEW CHIPS WORK WHEN SLOTTED INDIVIDUALLY INTO EITHER AVAILABLE SLOT.ALSO ALL COMBINATIONS USING BOTH CHIPS ALLOW THE MACHINE TO WORK BUT STIIL ONLY 256MB.CAN YOU SUGGEST WHAT STEPS ARE AVAILABLE TO NOW PLEASE.THANKS