RE-900 Build with OS and Monitor

  I originally wanted an i3-2120 and an 7850 but changed it to a i5-3550 ivy bridge and a XFX Radeon 6870. Later on when I have enough money, I wanted to upgrade the GPU to an GTX 670 or GTX 680. What kind of Power supply should I get? I heard that the GTX 680 needs a 550W power supply, what does the GTX 670 need? What's the power difference between the GTX 670 and the GTX 680?

Can you give me some options for some power supplies, with some links? Is there anyway to cap the price of the PSU around 60-75 dollars? I know it's kind of unrealistic but I'm on a really tight budget. Should I just get a 500W power supply for the i5-3550 and the 6870 and then upgrade my PSU and GPU later when I want to? I think that's the better route now that I think of it, I just don't want to remove and insert all the wires. But, if it must be done I will do it!