Well, the answer to how many hosts/VMs a basic GigE based SAN will scale to is... it depends! Workload, types of VMs, storage IO reqguirements, etc will all play a huge role. Most vendors should be able to run some tools in your environment to get an exact read of how many IOPS your dealing with. From there, they should be able to properly size something for you.
Having a lot of experience in these types of setups, I would wager a guess (and I'm plucking this out of the air based on gut feeling and experience) that a basic Gig-E based SAN should be able to support a solid 75+ VMs (<10 hosts) in an average environment (mix of SQL, exchange, file and print, etc), before 1 Gig ethernet starts to become a serious bottleneck. I've had the storage experts from both HP and Dell tell me in <50VM environments that Gig iSCSI should be more than enough. In fact, in <25VM environments, they even recommended 10K SAS instead of 15K.
Best practice for network config is a 1:1:1 ratio of controller ports to NICs on your host to vmkernel ports. So on a 4 port controller, you would want 4 NICs per host dedicated to iSCSI, each bound to 4 vmkernel ports.
There's advantages and disadvantages to any RAID level - you'll always sacrifice something. As requirements dictate, you might need different volumes with different RAID levels. Some newer SANs will automatically manage this for you. With Compellent, for example, you don't even touch RAID settings, it does it in a super smart way for you (always writing to RAID 0, alread reading from 5, customizable of course).
HP's are great solutions, but you'd be doing yourself a disservice if you didn't see offerings from other vendors as a comparison. SAN tech has come a long way in the past couple years, so do yourself a favor, and see what EMC, Dell Eql and Dell Compellent have to Offer.
