I’ve found some good resources for building raised garden beds.

Just a few notes: avoid pressure treated wood unless you have a plastic liner to cover the inside walls or infuse the wood with a penetrating oil such as AFM Naturals (Which is sustainably made from plant base.) Eartheasy says that another alternative to pressure treated wood is choosing naturally rot resistant breeds, such as red cedar, black locust or redwood.Of course, brick is another option and you may be able to do it even cheaper.

Raisedbedgardenigtips.com has a great overview of the benefits, tips and resources.
The DIY Network has a very low-tech version which will work just fine. Landscaping.com has a guide with a FAQ.
Sunset.com has a full materials list with relative prices. Raisedgardenbeds.com is a pretty expensive option, but they have some great landscaping and garden design ideas.

Tips and Warnings from ehow:

*The worse your soil is, the higher you’ll need to build your beds. If your soil is rich and loamy, sinking railroad ties a couple of inches into the soil will do. If you’re starting with hard red clay, you’ll want to go up at least a foot, fill with a couple of inches of gravel and sand for drainage, and then fill with good soil.

*Sinking posts (landscape timbers work well) at the inside corners of the beds and constructing the boxes around them will anchor the beds more solidly.

*Warp will happen eventually, and it will follow the wood’s natural grain. To minimize this, position boards so that any visible curve is on the outside (the side you’ll be nailing). That way, when the boards warp the nails won’t pop out.

*You can buy premixed soil – soil that contains a good balance of sand, peat, and organic matter like compost – at garden and hardware stores (it’s usually called garden soil, potting soil or topsoil), but you can go through a lot of 25- or 40-lb. bags if you’re filling a big bed. Consider ordering it (look under “soil” in the yellow pages) and having it delivered, especially if you don’t own a truck; it’s usually less expensive and it’s certainly easier.

*If drainage is a serious problem in your yard, you may want to have a contractor help you with the planning.

*Pressure-treated wood is infused with chemicals, some of which are toxic. Though the threats are debatable, a penetrating oil finished is recommended by the EPA to use on pressure-treated wood that may be exposed to children or pets.