While there is a lot of great information available on Trulia, determining the dollar value of a house requires significantly more than an on-line search.
It also depends on the reason you need to know the value. You did not say whether you wanted to sell the house or if you need it for another reason such as insurance, or refinance, etc... In that case, you might want to consider hiring a licenced appraiser.
Most agents are happy to discuss your needs and develop a "CMA" for you. A "CMA" (Comparative Market Analysis) is a detailed report that explores how your house compares against similar (not necessarily the same) properties in your area that either currently available for sale, those that have sold or are in contract, and often times properties that did not sell. An agent might also provide overall area sales statistics as well depending on whether good data is available in that region. Agents will spend a great deal of time and thought developing the CMA and it usually requires a visit to your property in order to be able to do the best job.
Additionally, agents are in the field everyday looking at properties and putting new deals together. We have a "feel" for what is happening in the market right now vs. looking a stats that are old news. For example, properties that are reported as sold were actually the result of a deal put together anywhere between 45-60 or more days before they actually closed. No, none of us can predict how much the house will sell for if it sits on the market for months at a time, but we can tell you what buyers are doing today.
To sum it up if you are considering a sale, talk to a local agent who will provide you with both a CMA and can discuss the current market with you in order to give you an idea of how to position your house in the marketplace in order for it to sell in the time frame you need.
If you do have an agent come out to do a CMA, but choose not to sell, please be kind to that agent. Be honest about what you want to do and if they were helpful, consider referring that agent to someone else who needs to buy or sell, and remember them when the time is right for you to make a move. You will really make that agents day!
Best of luck!