Hi Jeff,
There are a whole slew of tools out there than can help you. A lot depends on what level of accuracy you need. If you are dealing with an very precision, narrow or broadband RF signal, you may need to get a 3-D field solver. Ansoft, is one example of many big names that have a variety of tools that can help out. (google spicelink or HFSS for an introduction)
If your design is lower frequency and Transmission line accuracy is not as critical (only you can decide this), you could probably get away with a simple strip-line/micro-strip calculator and so forth. here is a simple example website:
http://www.technick.net/public/code/cp_dpage.php?aiocp_dp=util_pcb_imp_calculato...though there are many others.
A lot depends on the accuracy you need and how much time and money you are able to invest.