Hi Matt,
The R510's are a great product, you just got stuck with the wrong back plane.
As for iSCSI SAN's - great option, but what are the specs of the unit you are considering? MD3200 Series? Equalogic? EMC? etc...
Knowing that would allow me to give you some further insight, but yes, you are definatley on the right track by moving to an iSCSI based SAN!
Cheers!