Adding nVIDIA card to desktop

Hi everybody,
 
I bought the following desktop solution 4 years ago and now I need to add an nVIDIA card for scientific calculations with CUDA.
 
 - NZXT, H2 Classic Silent Black Mid-Tower Case, ATX, No PSU, Plastic/Steel
 - HEC, X-Power 585 Power Supply, 585W, 24-pin ATX12V, Dual 80mm Fans, OEM
 - GIGABYTE, GA-Z77M-D3H-MVP, LGA1155, Intel® Z77, DDR3-2400 (O.C.) 32GB /4, PCIe x16, SATA 3Gb/s RAID 5 /4, 6Gb/s /2, HDMI + DVI + VGA, USB 3.0 /4, HDA, GbLAN, mATX, Retail
 - INTEL, CoreT i7-3770K Quad-Core 3.5 - 3.9GHz TB, HD Graphics 4000, LGA1155, 8MB L3 Cache, 22nm, 77W, EM64T EIST HT VT-x XD, Retail
 - ARCTIC, Freezer 7 Pro Rev.2 CPU Cooling Fan, Socket 1155/1156/1366/775/939/AM2/AM3, Retail
 - G SKILL, 32GB (4 x 8GB) Ripjaws X PC3-10666 DDR3 1333MHz CL9 (9-9-9-24) 1.5V SDRAM DIMM, Non-ECC
 
I have Ubuntu as OS.I don't really care about using the discrete card for video output; in fact, I plan to keep the integrated Intel card for output and to use the nVIDIA card only for CUDA.
Are there any hardware incompatibilities for adding a discrte card to my desktop? Would it be difficult to install the card myself?
 
So far, I've been using my laptop with a GeForce GT 555M, but I need to improve the performance by a factor 3 or more.
What card do you suggest? My budget is below $1000, but, of course, the cheaper the better :)
 
Thanks a lot for your help!

Yes, you technically can use a 4-pin molex to create an available PCI-E 6-pin connection. However, due to the way power is regulated on each rail, it's not a recommended long-term solution.
 
Just for reference, some other user experiences with such a setup. 
 
https://bitcointalk.org/index.php?topic=51740.0 

You are correct...If it's short term, it MIGHT be passable, otherwise the absolute best solution would be to purchase a quality power supply.