Frank,
Personally, I would go with a Dell T610, although you will get arguments on here that this is too much. The 610 (by Dell's mouth to my ears) is the closest thing to the famous PowerEdge 2900.
Of course anything down to the 310 is fine, depends on how much you want to spend.
I am a big fan of having TWO separate virtual drives, one for the OS and one for your data, but nothing wrong with going RAID1, RAID5 or RAID10 (nice RAID) on one drive. Please note there is a difference between partitions and drives.
It is nice to be able to back up your data "drive" and know that if you need to you can reinstall your OS without having to worry about touching your data drive. Of course, you can accomplish the same thing with partitions, but better off to have them split.
If you go with Small Business Server 2008, it has to be 64-bit, because Exchange Server must be 64 bit. And, as you noted, depending on the motherboard and amount of DIMMS, you can get up to 64GB of RAM, which I can safely say that even for Sandeep, is too much.
12GB is fine, but as I will point out later in my state of the SQL standard address, the more RAM, the more SQL can use, which DOES give you better performance.
Note that SBS 2008 Pro COMES with Windows Server 2008 which unlike two copies of SBS 2008 CAN be put on the domain and still allow SBS to be the DHCP server and domain controller. You can then either back up to Windows Server 2008 in real time and have a system ready to go if necessary. The other thing it can be used for is a Terminal Server, which would be nice depending on if you have multiple users remoting in at the same time. SBS can be the only server on the domain.
Small Business Server gives you:
Exchange Server 2007 easily upgraded to 2010. The nice thing about that is you can archive all email going in and out which is or one day will be HIPAA necessary. Also, any emails sent to users on the network are not going out over the Internet and are, therefore, compliant.
SharePoint: It is impossible to explain the advantages of SharePoint. You will see if you have it.
Remote Web Workplace: The ability to log in easily to any machine with security that is second to none, although with SBS 2008 or regular 2008 will allow an SSL version of RDP, which is just as secure and extremely fast. Also, unlike the past OS which would allow connection to one computer such as the server and not to another client WITHOUT port forwarding, with 2008 RDC, you can log into any client simply by name. This can be done with either SBS or regular 2008.
Group Policy (which all servers with Active Directory have) will allow infinite possibilities of managing the entire network from the server. Just as an example, I just can't deal with my staff turning the entire desktop into a photo album or the latest album cover of the Rolling Stones. One click in Group Policy and that ability is no longer possible
I suppose 2011 would be the way to go. I just am stuck on 2008 and would be less able to help you if you need it.
Sorry for the long post.
Remember, unless you have full versions of SQL, which can take advantage of more memory, Express will still be limited to 1GB. But, if you upgrade (which is possible), then you can take advantage of more RAM. Of course, if you have SQL Server 2005, you would have to limit it or it would take up all your RAM.
