> the vm's will be mostly windows 2008 (exchange, a few sql servers, file servers, 2 dc's, general purpose servers, etc) and a couple of sco unix erp servers. i want to make sure that the shared storage can handle the io load of the exchange, sql, file, and erp servers
Really you need to benchmark the IOPS load on your current infrastructure using PerfMon or Microsoft's MAP tool. Or, if capacity isn't such an issue, go for a 15k SAS model like the PS4000XV.
Bear in mind when virtualising that a machine (and/or VM) will be needed for Virtual Centre Server. Also steps need to be taken to ensure the 2 DCs are never on the same physical host.
> we originally wanted to stay away from iscsi to avoid purchasing expensive network switches and doing the network tuning required for iscsi
iSCSI is simply SCSI over TCP/IP, there is no tuning required. If using ESX software initiator, the switches must support (and have enabled) flow control, but that's it. A couple of Dell iSCSI optimised switches is probably the way to go, to keep things entirely seperate.
> since we don't plan on have more than four esx hosts
If you can keep to three hosts, and don't need vmotion, the Essentials or Essentials Plus licensing will save a lot of money to put towards your iSCSI network.
HTH