I'd suggest going with a RAID 1 SSD setup rather than RAID 1 with SAS drives or Raid 5 with Sata drives if storage space is not an issue. You'll see significantly improved I/O performance with the SSDs. Make sure whichever RAID controller you purchase is compatible with ESXi as well. Dell software based RAID controllers such as the S100 and S300 are not supported.
RAID5 works great for reads but is often slow on writes. So it really depends on your usage. I suspect from the sites you host, you will see a lot more reads than writes, so RAID5 will probably work great for you. Just be sure to keep an eye on the RAID status to be proactive on replacing a failed drive.
If your going the Dell route and colocating, I would look at the PowerEdge R320 minimum. This would give you the option to have hot swap hard drives.
Id also recommend that you get the iDRAC 7 Enterprise for remote management.
You could also look at dealing with a system builder and getting a Supermicro server built. If you want to give this option a try on pricing, I recommend Clark from Orange Sky Technologies ; [email protected]