The amount of research you do before buying a tennis racquet should correspond to how well you play. Buying for a Beginner should provide enough information to choose a first racquet. If you want to develop a more thorough understanding, the Racquet FAQ, Racquet Glossary, and How to Choose the Best Racquet for Control and Power should get you well on your way.
The first time I bought a tennis racquet, making the choice was pretty simple. As I recall, I had $12 to spend, and the store had perhaps seven models from which I could choose, all with the same head size, the same profile, nearly the same weight, and the same composition: wood. Now, the variety of head sizes, profiles, weights, and compositions is almost endless. It's a lot harder to choose the right racquet from among so many options, but almost any model you might buy today will be a lot easier to use than even the very best of the old woodies.
Amazingly enough, you can still get a racquet for $12. It will be made of aluminum, factory strung, usually with a medium-wide profile and a mid-plus to oversize head. The big discount chains always have several models under $20. If you just hit around casually now and then, you probably don't need anything more, but if you're serious about becoming a better player, you'll probably want to invest at least $70, more likely over $100.
In deciding where to shop for a "serious" racquet, you have three main choices. A local tennis shop will generally give you the best service, including the all-important option to demo several models. Some pro shops have excellent prices, but you might spend even less at one of the big mail-order or online stores. You can also check for sales at large, general sports stores, although their tennis expertise will generally lag far behind that at a pro shop. The quality of stringing at mail-order houses and general sports stores varies. Pro shops are usually more consistent.