I am looking to build a computer with an Intel i7 2600 (no k) CPU. I don't want to overclock. Is an aftermarket CPU cooling system necessary if I am not going to overclock, or is the stock fan/heatsink sufficient?
The stock cooler will provide minimal cooling and keep the cooler within heat spec, while doing so loudly.
You can pick up a Coolermaster Hyper 212 plus for around USD $25 with a quieter fan and more effective cooling. It is a great cooler for the money. It does a better job with its single fan than my H50 with 2 aftermarket fans in push/pull (total cost of near $100).
Depends on what you plan to do with if, if your going to do heavy gaming or folding at home you will want to go with a better cooler, if it is just a basic computer for you the stock cooler will be more then enough in a well vented case.