I think 24 drives is much better than 3 or 4 drives in a small RAID. Also I was taking a look at some drive performance numbers and when it comes to 7.2K SATA drives they only deliver about half of what I am used to with 10K SAS drives, so only about 75 IOPS. It is recommended that a single VM would have 10-40 IOPS. If you are running a DB, you will need more.
With the Dell 2100, assuming RAID6 with a single hot spare, you would be looking at 1,575 IOPS that would support around 40 VMs. In comparison, a 4 drive RAID 5 would support around 8 VMs.