To call a WCF Web Service, your code needs to have a config file. To do this in Sharepoint 2007, you need to use the web application config file so you can edit the web.config file of that web application adding the bindings and the client sections to the system.serviceModel section making the web.config file will look something like that. Add ServiceModel seciton after configSections
The way I did is to create a WCF in VS 2008. Hosted and create a webpart in vs 2008. Use the same way to add service reference and write some code. Remember always catch excpetion in webpart otherwise the whole page will not work. Deploy the web part and modify the web.config to add ServiceModel.