Check out the folks at whole9life.com - http://whole9life.com/2011/01/paleo-poor-your-guide-to-the-grocery-store/ (WFS). They have a list here of what their recommendations are as to those that tend to contain the most chemicals (non-organic ones that is).

The list is here

If you Google "foods with most pesticides" you'll get some consistent lists of foods that are high in pesticides, I would use that as a basic list of foods to buy organic.

Check this out. It's the environmental working groups list of dirty and clean fruits veggies. [PDF warning]

Besides the listings of foods that are most pesticide-contaminated (a perfect way to start), there is also another consideration: the proportion of foods that you eat. This is how experts came up with the idea of recommending that milk, tomatoes (including ketchup), potatoes, apples, etc. should be prioritized as organic for kids - because milk, tomatoes (and ketchup), potatoes, apples, etc. are what the average American kid consumes.

That is, say you buy apples (which retain pesticide rather well) maybe once a quarter at most, but you eat a few sweet potatoes every week as part of your training; that is, frequently. In that event, it could be worthwhile to evaluate whether it was better to put money toward organic apples, or toward organic sweet potatoes (or maybe better yet, just forego or eat less of the conventionally-raised apples and buy more organic sweet potatoes).

Hope that makes sense.

Pesticides aren't the only problem with non-organic foods. They also have many more nitrates, due to liberal use of conventional fertilizers. Nitrates may turn to nitrites when chewing and nitrosamines in the GI tract, where they may increase the risk of certain GI cancers. On the flip side, consuming nitrates may also lower your blood pressure.

Adding a low-dose fat soluble antioxidant and a low-dose water soluble antioxidant together may inhibit nitrosamine formation. I use 12 IU vit E along with vit C with meals that contain lots of nitates - like beets or spinach.

Moreover, organic and biodynaminc farms, over time, should produce crops with higher mineral contents, as the scourge of monocrop agriculture wears off.

I buy organic everything because in addition to reducing my personal pesticide exposure, I like to "vote with my dollar" with regard to pesticide use in general by farmers. Even if I don't live right next to it, someone's getting poisoned. I think all organic also means all non-GMO.

Historically speaking, we spend a small fraction of our money on food. What better investment could there be?