It would be better to use the 120mm fan on the outside of the case with the radiator on the inside, while still using the fan as an exhaust fan. At least this way the hot air is leaving the case, not feeding the PSU and GPU.
I wouldn't recommend getting a boxed solution though. (by boxed I mean an all in one kit) Different manufacturers have different strengths, and an all in one solution has very many weaknesses. You will likely get better temperatures from one of the recommended heatsinks
than an all in one watercooling setup.
The Megahalems is a great heatsink, with a Scythe S-Flex or Nexus fan will keep your system cool and quiet.
If you really want watercooling I recommend:
1. A Swiftech Apogee XT (They are compatible with LGA775 1156 1366 and have an adaptor for AM2/3)
2. Thermochill PA120.1 & G3/8 1/2" fittings (The fittings are sold seperately)
3. Liang D5 or Swiftech MCP655 (Not the -B version, speed control is VERY necessary)
4. About 10 feet of 1/2" ID Tygon tube (So you can mess up and still have some extra)
5. A 'T' fitting to fill the system (with a T-line)
6. Hose clamps