Hi,
A gigabit link is just a gigabit link, so these can be quite easily put to the limit by any ISCSI storage system. Do not expect to get major enhancements from the flow-control or buffers, it still will not get you past 1G. It only postpones the drop of packets (for some milliseconds), but then the drop will still occur.
If you are worried about the performance, make sure to add as many links into the link-agg (trunk) as possible, or consider the 10G option (if you did not buy them yet, make sure to order a version with the 10G ports for future use, the old 3500 could be equipped with a 10G module, the new 3800 must be purchased with the 10G ports onboard.
Since you will have dedicated 3800s for the ISCSI, consider configuring the entire switch to use only 2queues instead of the default 8, this will increase the available buffer for the 2 queues (1 queue is not possible).
This requires a reload of the switch, so better do this at install time.
Best regards,Peter