Generally, you can get the most bang for your buck with "build-you-own" generic machines once called clones. (Have the store build it, though.)
Go to Frys.com, or a local store, or the local generic "computers plus" no-brand computer store with a good reputation, and get a good case, power supply, memory card reader, 3 tb hard drive, or two, for convenient backup, perhaps. Make sure it has at least 3 or more processors, and is 64 bit capable (motherboard/memory/cpu) Add a good flat screen monitor, and the most memory possible, and perhaps a SSD Solid State hard Drive (for fast OS and software loading).
Make sure the power supply is very quiet (perhaps even water cooled CPU), and be sure you have USB 3.0 ports and a firewire port. Compare the price for such against a mid-range graphics machine, and you should be quite happy with the result.
The os: probably Windows. Everything apple tends to be twice as expensive. I'd try to get W7, and then leapfrog to W10, which will be the next version. W8 stinks, and W9 will never be released. Win10 will have many of the lost features of Win7 restored. Make sure you install anti-virus software and spyware removal software. Tell your machine builder to NOT put any "Cramware" on your machine (software that you don't ask for specifically, usually with a downside.) Make sure you have drivers on CD/DVD, and the windows OS. Having it on a seperate recovery partition on your machine is NOT adequate. (If your hard drive crashes, you need to be able to restore it, and more so if the computer itself is lost, broken, or stolen.)
Onlinebackup is NOT adequate for a prolific photographer. IF you shoot more than I gigs a day on a regular basis, you need a secondary or external hard drive of sufficient size to allow backups.
$2000 is plenty to accomplish all or most of the above.