AnandTech has posted a fairly extensive set of benchmarks comparing the iMac G5 1.9 GHz vs iMac Core Duo 1.83 GHz. Interestingly, this article also tests the Core Solo 1.83 GHz configuration, by deactivating one of the iMac Core Duo's cores using the developer tools.
Their conclusion is one that I agree with: If you need a computer now to do very basic things such as what can be done with Apple's iLife and iWork applications, the iMac Core Duo is an excellent and fast solution. However, if you need anything substantial that is not a universal binary then you'd better think twice. I personally will be waiting until at least late 2006 or possibly even 2007 before I buy my first Intel Mac. Rosetta translation is a significant achievement, but it can only take you so far when you want to do real work with your applications. Don't forget about drivers for your third party hardware either, since existing drivers may or may not work with the new Intel machines.