I'm not sure what P03 is, but it you mean POP3 then you've got a 5-10 split. Gmail does support POP and e-mail forwarding, meaning you can use your own e-mail client such as Outlook or Thunderbird. Yahoo, on the other hand, does not give you that functionality unless you have Yahoo Mail Plus...their paid service. Thus you'll be able to use the client for two accounts, but still have to check Yahoo separately.

In regards to setting the e-mail client up, you can click here for a tutorial on Thunderbird, and here for a guide to using Outlook Express.

Hope this helps,
John