I was curious what an "Entry Level Server" costs.
This server was $1,395.00, when compared to the Lenovo $299.99, it's quite a bit more!
So the thousand dollar question, why go for Small Business Server?
Your all-in-one network solution, designed and priced for small businesses with up to 75 users. SBS 2011 Standard delivers enterprise-class server technology in an affordable, all-in-one solution. SBS 2011 Standard helps protect your business information from loss by performing automatic daily backups. Additionally, it allows users to be more productive with features such as e-mail, Internet connectivity, internal websites, remote access, and file and printer sharing.
In addition to the server software, the hardware in a "File Server" is actually different than that in a desktop, particularly a low end desktop. Servers are designed to work 24 x 7, and essentially work "Forever". They usually have "Redundant components" (Like dual power supplies, RAID ARRAYS), and other features.
Now, specifically looking as Business Rules and Client Server, the $299.99 PC is probably just fine!
I do have a caution with the E3400: http://ark.intel.com/products/42772/Intel-Celeron-Processor-E3400-%281M-Cache-2_60-GHz-800-MHz-FSB%29
This CPU only supports 2 threads, and client server will be "Choppy" if you load more than a few users on it.
The "Entry Level Server" has a E3-1220 CPU: http://ark.intel.com/products/52269/
(4 Thread Solution)
For $230.00 more, the dell server can be configured with an E3-1270 CPU: http://configure.us.dell.com/dellstore/config.aspx?c=us&l=en&cs=04&kc=&oc=besw5t2
(8 Thread Solution).
Intel has a nice comparison page that shows the 3 CPU's compared to each other: http://ark.intel.com/compare/52276,52269,42772
It took me a while to figure out the "Inexpensive Desktop Market". For some reason, they won't say this is the "Cheap & Junky System". Looking at the Lenovo Series, the M48 uses "Lower End Components" (A safe way to save money), but I get the impression that they also use "Cheaper Components" (A not so safe way to save money).
Take a look at the hard disk, Dell sells: 300GB 15K RPM Serial-Attach SCSI 6Gbps 3.5in Cabled Hard Drive [$449.00] Why is this hard disk (All by itself) more money than the entire M48! It's roughly the same same storage. (PS: That is the drive that I go with)
So the answer to the question, and the reason to go with an inexpensive desktop is to save money upfront. The hope is that none of the components will fail, leaving you without a server. Having Mirrored drives + a good backup means that when the server fails, you can buy another one to replace it.
The worst case scenario is repeated downtime, rebooting of the server, etc. And of course loosing data from the hard disk. The business question is what does 3 days of downtime cost my business. (Re-Order, Re-Install & Recover backup after a disaster).