Generally the advice is to not override the functionality of an existing button, since it makes installing other solutions able to stomp on changes you've made. Probably better to use the Ribbon XML to hide that button, then create a near
identical custom one that executes your web resource. Your other option is to use the Ribbon XML to override the functionality of the existing button. If you download the CRM 2011 SDK, there is a guide in the walkthroughs\ribbon section I believe.