Is it ok to add a 1gb chip to the 512mb chip already in the imac? I'd probably lower the current chip to the lower slot and add a new 1gb chip to the upper slot....is this ok for this type of mac? I think 1500mb of memory should be ample for me and don't want to have to pay $200 for 2gb, 1 gb chips each to balance the load. Please advise. Thanks!