don't go with the dells for a giant setup. (have had the dells and I use the evga's now)

Gator-rex is steering you correctly.

if you are real you want the evga super nova 1300.

 runs 3 overclocked 10 year warranty and cost is 195   that is 66 dollars a s-1.

two of these are 388 usd that is 6  s-1's units.    

three  dells will not over clock 6 s-1's safely .   Shock Hazard  fire hazard   not much resale value.


tell you what get 2 evga 1300's and setup 6 s-1's  over clocked to 393 or 387 freq.   get 3 dell and try to do 6 s-1's over clocked.


here is a link for the evga's

http://www.rakuten.com/prod/evga-supernova-1300-g2-1300w-power-supply-110-v-ac-220-v-ac-input/249973555.html

next you need a lot of switches.  get one of these to start

http://www.rakuten.com/prod/tp-link-tl-sg1016-10-100-1000mbps-16-port-gigabit-19-inch/213928097.html

that lets you do 1 in 15 out or 15 s-1's


oh you need two internet lines not one if you go over 250 or 256.

a router taps out at  amax of 256 including the router     192.168.0.   1- 256     these 1-256 are the most you can do on 1 router.

with that many machine 278 you want a second fan on every one  here is a link to the delta

i used  http://www.ebay.com/itm/151276722479?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649


mount this fan with 4 pipe cleaners and 4 nylon nuts to allow the fan blades to clear.