People always seem to grossly over estimate power requirements. A quad core and GTX 260 uses just under 300w at the wall in full load. 500w+ is plenty of extra buffer.. you don't need a 700-1000w power supply.
Here's a reference with a more power-hungry CPU.. the i7 965:
http://www.anandtech.com/video/showdoc.aspx?i=3501&p=9
Your system would be FINE on either a Corsair 520w or Corsair 620w (both modular power supplies made by SeaSonic - one of the best PSU makers). I've run a Q6600, 8GB of memory, 8800GTX & 9600GT, 2x Raptors, 2x 7200rpm, SSD, 3x optical drives.. with the CPU, ram, and videocard overclocked. The Corsair 620w has been more than enough for 2+ years. All this talk of 700+w minimum is for multiple GPUs or.. power supplies that aren't made by quality manufacturers. The Corsair 620w is perfect.. and the 520w will be enough. Water cooling doesn't use a lot of power.. most pumps are at most 20w.