I'm not an expert at this, please check my answers.

I'm not sure about iOS 4 and the 3G, I believe there were some problems, I don't know if they have been addressed or not.

First ask yourself what is actually wrong with the laptop, if the hard disk (HDD) is still ok then you should investigate repairing/replacing the laptop but keep the original HDD or salvage information off it. If you can manage it, this will solve some of your problems.

As for having to sync with another computer, you will probably loose the music but you should be able to keep everything else. If you purchased the music through itunes you may be able to save it too. A phone can only be associated with one iTunes library at a time, but if you're not mucking with music and wot-not then you may be OK. Just dont overwright all your girlfriends contacts etc on the laptop, if you do, iOS4 will be the least of your worries!

If you log in  a different user on the laptop, it may make your life a
little easier and safer. You should then be able to authorise the computer to your
iTunes account, this way you will be able to transfer all your apps from the phone to the computer, and back again later.

First thing to do would be to connect to the laptop and backup the phone, it should do that, on anyones account (I think), without changing anything. Restoring from the backup later will then restore all your settings and things like, email settings, contacts, and photos.

From here the outcome depends on what you've done so far, if you've created a new user on the computer and backed everything up correctly, then you should be able to restore to almost the state you had it.

Not exactly a complete or concise answer I know. Hope it gives you a little more of an idea though.

AndrewNZ