200MB DICOM file is about average size (if not compressed), it should be no problem at all to load two of these. I usually recommend to have 10x more memory space for an application than the raw input data size - then you don’t have to be concerned about memory usage. So, for 2x200MB input data, 4GB memory should be enough. Make sure you configure virtual memory/swap size in your operating system accordingly and of course build a 64-bit application.