[CentOS] SSD drives for the OS - 1 or 2?

Preparing to build a small replacement server (initially built in 2005) and normally for the OS I would buy 2x500GB drives and deploy in a RAID 1 configuration. Now we have SSD drives available - does just a single SSD drive offer the same reliability or is there advantage in deploying two in a Raid 1 config?

Also, what form factor / interface is best for the SSD OS boot device on a server M/B? Anything I should be looking for? It seems most SSD are in laptop drive size, shape and interface - thus not a good fit into a server motherboard unless I'm missing something. - what devices are you using?

Thanks in advance. Rob