well, they will be wiped from your phone, but you won't really "lose" them. once you install a rom and boot up, you will have to re-establish your gmail account. once you do, most roms will automatically download all of the apps that you previously had on your phone. this is because you have now re-established your "link" with google, and they already know who you are and what you've downloaded.
as for contacts, as long as you have your contacts synced with gmail, those too will reappear once you've re-established your gmail account on your phone. in the initial set up after rom installation, you will enter your gmail username and password, and then google will sync all of your gmail info with your phone.