Hi. I have a nokia 6600 too.
The firmware version is 3.42.1.
I am interesting in making an upgrade.
Do I have to pay for this upgrade???
Where I can get a firmware version list for my phone??
The latest is 5.27 and you can get it from your nearest authorized Nokia service center. Start looking for the location here (for example): http://www.nokia.com/nokia/0,,42096,00.html
upgrading firmware is usually not necessary. i use the earliest version and it works fine except for some errors that can be easily overlooked. But if your other accessories are incompatible with your phone, you might as well try to upgrade it and see if it works out.