What will happen...?

So I've been creepin in the shadows since upgrading my 2G iPhone to 3.0. JB'ing my phone is very appealing because I want to do some graphical changes and some of the apps on Cydia are appealing. I have been looking for these answers (even regular google searches) but I have been coming up empty handed so excuse my ignorance please!

My question what will happen to my current apps / settings when I JB my phone. I'm assuming my phone goes to 'standard' everything, as if it was restored to factory settings. Additionally, once I've JB'n my phone, will I still be able to purchase apps / music through iTunes?

If theres a page or thread that covers that, please just point me in the right direction! Thanks!