With 6GB of physical RAM I’m surprised that you can load a volume of 512x512x545 voxels at all. I would recommend minimum of 16GB RAM, an Intel Core i7 CPU, and a computer not more than 3 years old.
Since you will only create a few models, it should be no problem to invest a lot of time in the segmentation process. As you also want high accuracy, it is probably better to not use curves (which would only give you vessel centerline curves and not diameters), but simply set an editable threshold range and then paint the vessels in 3D or slice views. I’ve uploaded a short video tutorial here:
