If you want or require the ultimate in speed and reliability, go with SCSI.
SATA can get you performance near that of SCSI, and good reliability. This has (almost) nothing to do with the SCSI and SATA technology itself, and everything to do with marketing.
There is a wider range of reliability in SATA drives as most SATA drives are targeted at the desktop market. A few are targeted at the server market. SCSI drives are targeted exclusively at the server market at this point, and so they have specifications more-suited to that market.
It's hard to go wrong with a SCSI drive. It's easy to go wrong with a SATA drive.
For a good balance of speed, reliability, and price, use 10,000 RPM SATA drives that are specifically intended for servers. Check reviews first. There are some great SATA drives and some really awful ones.
As to whether or not your hardware will be up to the task - who knows? You haven't supplied enough information. 50 users doing what? Transactions/second is a much more useful figure than number of users. Even then - how complex a transaction, what are the usage patterns, etc.
I've been using dual CPUs in both servers and desktops for years now, and I wouldn't go any other way - although counter-intuitively, I think it's more important for a desktop than for a server. For a desktop, dual CPUs is a huge responsiveness win.
But I would look for a server using one of the newer chips that pack two CPUs in one package.