I faced quite some problems in configuring Pidgin / Gaim with Google Talk on Fedora Core 6. Here are the detailed steps for a no-brainer configuration of Pidgin for Google Talk:
1. Click on Account->Add/Edit menu
2. Click on Add button in the Accounts screen
3. Select XMPP / Google Talk as your protocol
4. Type your screen name which is your GMail login name without the ending @gmail.com
5. Type Domain as gmail.com
6. Ignore Resource
7. Type in your Password and check Remember password
8. Click on Advanced tab
9. Check Force old(port 5223) SSL
10. Check Allow plaintext auth over unencrypted streams (may not be required)
11. Change Connect port to 5223
12. Type talk.google.com in Connect server field
13. Click Save and you are done.

Note: Use the Gaim from Fedora extras. Do not install pidgin directly from other repositories.