Raised beds stay warmer longer into the fall and warm up faster in the spring. If you have soggy soil, they help keep the soil in the bed drier. However, if you already have dry soil, or live in a warm climate, you may not even need raised beds. (If you’re in a warmer climate, you might also look into "French Intensive Gardening" where you simply mound the soil in your beds, and don’t need to go through all the expense and trouble of building raised beds.)

But if you want to build raised beds and keep your garden organic, there are a few options. Definitely never use pressure treated or "wolmanized" wood – it contains arsenic, cyanide, or copper that will harm the plants and organisms you want to encourage. Don’t use vanished wood for the same reason – varnish contains chemicals and oils that are not good for your soil.

Cedar would last a long time against rot and bugs, but it tends to be very brittle. So if you’re going to accidentally hit it with a shovel, hoe, spading fork or other tools often, it may splinter or even break.

Redwood would be a better bet, but it can be expensive. If you don’t want that expense, plain untreated pine lumber is perfetly fine and will last 4-6 years. It’s true that you will have to replace it eventually, but it may be cheaper in the long run than replacing cedar often, or the luxury of redwood.

You can find some easy raised bed designs (that also minimize the amount of cutting you need to do) in an article I wrote at the link below.

http://home.comcast.net/~bcballard/garden/16-raisedbeds.html

ask at your wood yard or the place you intend to buy the wood from. they should be able to advise you as to the wood and the type of varnish best for the job.