Oliver, be warned, VPOs (virtual pipe organs) are very memory hungry due to the size of sample sets; for every rank of pipes there will be a sound sample in .wav format for every note; a sample set can be over 1Gb, it will depend on the number of pipes in a rank, typically 60, the number of ranks and the size of the individual files; I'm not sure of the reason for the latter, maybe duration of the loops, perhaps one of the experts can comment.
To sum up, 512Mb is not enough, except for maybe a very small organ, I get low memory warnings on a 1Gb PC when loading a set of 24 ranks, so far 2Gb is coping, until I get a bigger sample set!!