Iphone 3GS upgrade and jailbreak Help please~~

[nhkim85]

I have:

Iphone 3GS
jailbroken
firmware version 3.1.2 (7D11)

I believe the firmware I currently have is a "hacked?" version
because at the time I did the upgrade, Apple had blocked the 3.1.2 upgrade by simply downloanding a previous firmware and "restoring" on iTunes

I'm sure you get what my situation is and is probably quite simple for all you experts here lol

What I am looking to do is...
upgrade the firmware to the latest firmware and jailbreak it
(when I say the latest firmware, of course I mean the latest that can be jailbroken)

Is there a way to just upgrade the firmware without losing the jailbreak?
or would I have to re-jailbreak after updating?
is losing all the apps downloaded by installous unavoidable?

Thank you in advance
and please help~~~