What data will I loose when I do a factory reset of my Android phone?

Hi! I have a Droid 2 that I just upgraded to Gingerbread. I am not rooted. The phone was slowing down before the upgrade, now it is really sluggish. I am thinking of doing a factory reset and run the OTA update again. Then I guess I have to reinstall all of my apps. But the question is… what “important stuff” will be wiped out? What should I try to back up first? Is there any software I should be using to assist me? Thanks!