System Usage from Most to Least Important: Data sever, remote login, productivity/accounting software (MS Office, Quickbooks)
Are you buying a monitor: No
Parts to Upgrade: None
Do you need to buy OS: Yes
Preferred Website(s) for Parts: Newegg, Amazon
Location: Philly PA. (have a microcenter nearby)
Parts Preferences: none
SLI or Crossfire: No
Additional Comments: This computer will be on all the time. The main advice i'm looking for is the CPU. One hardrive is for the OS and software, the other two will be for data in a raid 1 configuration.
Windows 7 should be able to handle those sizes of databases and connected users - however the operating system doesn't offer much in the way of security. Server 2008 offers more security - but the price is going to blow the budget. Costs are $600+ for the software.
As for the CPU/RAM/Hard drives - You should consider the WD Black drives for the data drives, and my suggestion would be to skip over the RAID configuration - as the size of the databases would be easily restored.
I would configure the server with 3 drives - a blue drive for the OS, and 2 1TB WD Black drives as the data drives. Use SyncBack Free to backup the data - it can be configured to run daily automatically (backup first drive to second drive). If one drive fails, simply plug in another drive and copy the data. The 2nd drive is a full working copy of the original data.