I have to say that by building your own system, you will get far better results than with any kit, (though the Swiftech H2O 22501 kit is the best of the kits). It comes down to how much money you want to spend on it, and how much you want to mod your case. For the price of the Swiftech kits, you can build yourself a better system.
People have already mentioned DangerDen, they can be considered as the best providers of water cooling parts, (except their pumps, the Swiftech MCP600 is the best pump).
A good water cooling system is ~
Pump ~ Swiftech MCP600
CPU waterblock ~ DangerDen RBX/Maze4 or Swiftech MCW5002
GPU block ~ DangerDen Maze4
Northbridge block ~ DangerDen Maze4
Radiator ~ Hardware Labs BlackIce Xtreme 2
Tubing ~ Tygon 3603, 12.5mm I/D 20mm O/D
Reservoir ~ I made my own from a plastic bottle and two fittings
On the list above, I went for the Swiftech block, as it has a far better flow rate, and very nearly equal performance to the DD.
Temps are 30/31 at idle, 34/35 at full load @ 21 degree ambient.