I personally run both Ubuntu and Windows7 (don't shoot) and I like them both....Ubuntu does have a bit of a learning curve but it's not too bad...it's probably one of the most "user friendly" linux distros...
As to OpenOffice- I personally live in Excel, Access, and PowerPoint for work and I do not like the OpenOffice suite as much as I like the Office suite. That being said- OpenOffice is a great open-source alternative. As to Outlook v. Thunderbird - again, I personally prefer Outlook but Thunderbird is a great client. Firefox vs IE however- there's really no contest- Firefox is the better browser hands down...even on a Windows-based netbook.
The best thing to do would be to download OpenOffice, Thunderbird, and Firefox on your computer right now...they're all open-source and free so you can get a feel for them right on your current setup before you make a decision to switch.
http://www.openoffice.org/
http://www.firefox.com
http://www.mozillamessaging.com/en-US/thunderbird/