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).
The stock cooler should be fine. I wouldn't bother installing an aftermarket cooler until you at least try out the standard cooler to determine of the temps are too high or the fan is too loud.
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.