Holy crap!!! Please don't spend that much $$$
Buy a Dell Precision T5500 machine on eBay with a dual-CPU setup and upgrade it. I'll provide the recipe below.
THE ONLY BAD THING I WOULD CAUTION YOU ABOUT WITH DELL T5500/T7500 MACHINES IS THAT THE SECONDARY CPU RISER SUCKS AND IT VERY VERY FRAGILE. IF YOU BEND A PIN IT CAN RUIN THE RISER AND THE MOTHERBOARD... JUST BE VERY CAREFUL WHEN YOU SLIDE IT BACK IN TO MAKE SURE THE RISER IS ALIGNED PROPERLY IN THE TRACK, IF NOT YOU CAN RUIN IT. (don't let that scare you, it's not that hard)
Buy a Dell Precision T5500 on Fleabay for about $300-$400. I searched "Dell Precision T5500 dual" in the box and hundreds came up. (like this one:
http://www.ebay.com/itm/Dell-Precis...1402259220?pt=Desktop_PCs&hash=item338c9a5f14)
Call it $450 shipped would be reasonable so far (remember you can sell what you don't use and you can find them for $300 just as easily as you can find them for $700, forget about the CPUs, just get ANYTHING with dual-CPUs, the cheaper the better.)
The T5500 has 9 memory DIMMs (6 on mobo and 3 on memory riser). I like Wintec brand memory, it's cheap and has been very reliable for me. We are buying different sizes because we want the 6DIMMs on the motherboard to match what the 3DIMMs on the CPU riser have (so 6x 4GB and 3x 8GB = 48gb total but 24 each on the motherboard and CPU riser)
Buy two of these: (or similar)
http://www.newegg.com/Product/Product.aspx?Item=N82E16820161457R at $150 each so $300 total.
$750 so far - $450 for the box and $300 for the first block of RAM
Buy three of these:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820226414 that's $237 for that block of 24GB RAM... notice a theme of about $10 per gig of RAM (roughly)
(whatever you do please don't buy Kingston ECC RAM... only buy Mishkin, Samsung, Wintec (rebranded Hynix) or Hynix memory... it's worth it to pay for the good stuff)
So we are up to $987 so far for the box and the RAM.
Go to ebay and buy yourself "Xeon x5650" and buy a MATCHING STEP CODE lot of two cpus for less than $250.... like this:
http://www.ebay.com/itm/Lot-of-2-SL...T-s-/231190547193?pt=CPUs&hash=item35d407caf9
So now you are up to $1,237 for the box, memory and CPUs (while you are on NewEgg buy the Artic Silver $10 thermal compound so $1,247)
Then you need to decide on SSD's and HDD's... Right now you are well under 50% of your low end budget. you have a total of 6 bays in a T5500. usually they come with two optical drives (one reader one RW) a floppy or a memory reader and a blank hole (covered). There are two drive bays lower in the chassis as well. I've shoved up to 10x drives in there with RAID cards and using SSD's with power splitters... but you don't need that.
If it were me I'd pull the reader CD drive, spend $15 on eBay and buy the blank plate and a dell HDD caddy to covert a 5.25" bay down to a HDD 3.5" bay. I'd then go buy buy up 3x Samsung 2TB drives at $104 each:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822178391
which would bring your total up to $1,559 plus the odds & ends
One of these for $15.95:
http://www.ebay.com/itm/Dell-Precis...264?pt=LH_DefaultDomain_0&hash=item338a84b228 if you can find just the inserts it'll be cheaper, i got lazy searching for part numbers.
If you are worried about air flow these are always nice to have at $6.95 each:
http://www.ebay.com/itm/Dell-Precis...167?pt=LH_DefaultDomain_0&hash=item3f287f5cff
and one or two of these for $15:
http://www.ebay.com/itm/Precision-T...734?pt=LH_DefaultDomain_0&hash=item3389d67a6e whatever you do... make sure it has the screws on the side. Email or message the seller to confirm this - it's a ROYAL PAIN IN THE A$$ if you get them without the slider screws...
So for another let's say $50 with shipping you have all your little pieces and parts so you are up to $1,609 with 3x 2TB HDD's that you can put into RAID5 on that machine and have 4TB of redundant space... If you are really ambitious I'd buy an extra 2TB drive so you have a spare on hand if anything were to die... add $104 for that and you are up to:
$1,709
Buy yourself a really nice SATA-III 240GB SSD like this for $189:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820167177
and you are at $1,898 all in... $1,900 for a 24-core (12-core without hyperthreadding), 48-gb RAM full on bad-ass server-grade machine that runs more than you would EVER need... it leaves you enough left over you can buy me a beer sometime...
If you have the crazy itch/need to spend more the next level up that I've configured for some funds (serious backtesting machines) is a dual-8-core box with 96 or 144GB RAM and that's about $3000-$5000, depends on if they go RAID card, what memory config, SSD's or 4TB HDD's, etc. etc. go big or go home... So if you need to spend $3k-$4k I can help you there as well but you don't need to ever have that in front of you... it should be used as a server, etc.
Good luck! I see guys spending $1200-$1500 all day long on these crappy 6-core i7 boxes with 24gb of ram. It frustrates me that for just a little bit more they could have something so stupid overkill it would last for 3-5 years... or probably all most people really need is a decent little Optiplex 990 like this for $700 (that's expensive IMO):
http://www.ebay.com/itm/Dell-Optipl...1367985414?pt=Desktop_PCs&hash=item4ad3064d06 and they will be off to the races... you can easily put a video card in that capable of running 4-monitors for $50. If you buy a barebones machine and upgrade you can do it all in for $600 and have it better than the one I listed.
Enough typing. Good luck!!