50-150 is a big variance to plan for, and if you were to dedicate 20mb/s (about what an IDE disk is) to each virtual, you would only fit about 6 per interface.

you may want to look at chelsio 10gbe.

also the per target iscsi options are brand new.. i never planning on having a lot of targets but I can feel your pain on having to enable Immediate Data on every target!