How to choose PSU

I highly doubt you need 1KW worth of power. Post your full system specs and we will help you pick one that will suit your needs.

General rule of thumb, most single GPU setups can be powered with a 500-550W unit with ~40A between the 12V rails, most SLI/CF setups can be powered with a good 700-750W unit with ~60A between its 12V rails.

A couple things to look for when picking a PSU, obviously make sure it provides enough power, but more importantly, there should NOT be a voltage selector switch on the back, that is a sign of a much older design, it should be atleast 80+ certified, preferably 80+ bronze or better. 80+ means that the unit is reasonably efficient and can pass a test at 100% load.

To further what hunter315 said, I prefer single 12V rails. It simplifies the build since you don't have to worry so much about load balancing the 12V sources with the 12V loads.

I also prefer going with a reputable manufacturer like PC&C, some Antec, some Corsair, etc. Picking a brand such as Ultra or Rosewill because it's cheap and fits your build requirements for power is a disaster (or at least a disappointment) waiting to happen. Pay more up front and buy once, rather than pay less and then RMA/replace/cry over blown components.

I'm kinda going on an all out configuration. i7-980x with HD5970, with an option to crossfire in the future. SSD as primary disk. 2x2TB hard disks in non-raid config. blu-ray writer with dvd writer, and a media card reader with a touch panel. That's why the 1kw of power.

Actually mainly I wanted to what exactly should I consider while buying a PSU, coz I simply don't know how to find out whether how a PSU is compatible with a case or not.

In most cases an ATX power supply will work with any ATX case. That said, some power supply/case combos can be difficult. Big cases such as the Cosmos your using may have problems with cables reaching. Other than that, you shouldn't have any issues. Sometimes with longer power supplies, smaller cases can cause problems, but your case should have plenty of room for even the longest PSU's.