C heck in your motherboard manual for compatibility. If the motherboard manual does not give clear information, check on the motherboard manufacturer website. If you decided that it's a better investment to change multiple components, then you get to do a bit more reading. Consider how you are planning on using the system, and how long you're going to keep it. If you are going to keep your system for a few years, you may want to purchase the most power you can afford for the time.
Parts become obsolete quickly.
If you purchase below the current standards, you may not be able to upgrade in a few months. Many web sites post technical information. These may help you find a category of processor and board that are close to your interests. Another good source is talking with people who have the same expectations of their system as you. Newsgroups list problems people have had with components and possible ways to solve the difficulty.