Jailbreak update data loss?

[monolith]

Ok, so I wouldn't consider myself particularly new to the jailbreak scene, but this is honestly the first time I've come across this sort of thing.
So, I have an iPod touch 3g pumping iOS 4.1, jailbroken. Now, if I were to upgrade to 4.2.1 in iTunes, then re-jailbreak, would I lose all my Cydia apps and their data? I like Greenpois0n, so I would use it if that means anything.
I went and purchased PkgBackup, but I've only kept that as a precautionary thing. Like, in case I would lose my data. So, will everything disappear?

[Chibichuba]

Yes, you will lose all your Cydia data, unless you make a Cydia backup account and store all of you data onto it, which I have heard, is a new feature. You backup app will work too, but I think a Cydia backup account is free. If you have a battery percentage tweak, that will stay there forever even if you restore.

[monolith]

Ok, cool. Thank you